First off, I've never competed in my life nor attended a school (there are no practical options nearby), so ostensibly I am just another internet self appointed know-it-all... However, barring a nagging injury (incurred through gardening, no less... ) I've drilled on and off with a longsword for a few years, and certainly count as a HEMA enthusiast (Guy Windsor's books on Fiore/Vadi are the primary source). As such, I - like most people actively participating or simply interested in HEMA - have decades worth to learn. I bias towards historic reconstruction (not re-enactment), though, and obviously HEMA and popular culture rarely mix well.
Now to the video.
Firstly, there's not a great deal of opening context. He muses on how a typical soldier may've fought in terms of style, yet doesn't mention the longsword by itself wasn't a frontline weapon - if we're primarily looking at the longsword/hand-and-a-half-sword, then it's ostensibly a side-arm at best, and certainly not a weapon you'd equip rank and file. Add a shield, however, and it becomes a different discipline (usually with a different sword. good historic sources on shield technique are, seemingly, frustratingly rare).
Re reach: he addresses the silly spins and wotnot (I disagree with him re the viability of spins), yet never mentions the most important aspect of reach when it comes to edged weapons - the thrust, the simplest, and generally most effective threat any combatant can offer.
To talk about fencing and not discuss the thrust is just bizarre to me, regardless of whether Geralt tends to feature it in a combo or not.
Re his final remark about using a pirouette successfully in sparring: what kind of approach to HEMA is that? 'It works in sparring = effective technique'? Again, no real context; are we trying to merge historic and fantasy forms? Compare them? Find a winner?
When he talks about the self-taught teacher's wide cutting style; yes, there was no one historic form, and if people want to synthesise forms on a whim, have at it. But then it is not HEMA at all, given in some ways it isn't just a martial art encompassing hand to hand, dagger, swords, bucklers, armoured and unarmoured fighting, spear, pike, and mounted combat, etc - HEMA is also arguably on the periphery of experimental archaeology in terms of a reconstructive living approach to period sources and knowledge.
The distinction is important because HEMA directly references forms and techniques that were proven in conflict (even if you're just looking at the disciplines that were arguably most used/effective in judicial duels) - they were disciplines to grievously maim a human being or outright kill them. A bias away from that to contemporary synthesis and development thus makes it a discipline defined by what works in a safe, non-lethal, non-life changing environment. Olympic sport fencers do not, for example, practise a martial art - it is a derivative (that's not an insult, btw, I mean in terms of lineage) sport, now shaped by its inherent safety and absolute focus on point scoring.
To 'fight like a Witcher' is, then, surely to fight like a sportsman. Throughout history the techniques that endure are the simplest, least intensive techniques which work systematically.
The rand overhead: obviously he has a legit HEMA parallel, but the danger of these kinds of demonstrations is that it's very easy for newcomers to see a given - supposedly authentic - technique and then take that as the 1:1 model of what is effective or how something is and was performed. It's always worth noting that, at least in HEMA, those kinds of cuts are rarely ever in a vacuum, i.e. not executed from a static position, but combined with a side/quarter step, or flowing from an upwards cut (or beat) which then leaves the sword 'chambered' for the downward cut.
Overall? Eh, it was okay. Not exactly a great contribution to modern fencing culture, but certainly by no means exactly bad. It is very much on a different path to the one I prefer, so it's not my thang.
These days so much is said about what was or is right or wrong about forms (or weapon types, or which culture and/or era's sword is 'better'... yeesh), and I've done some sniping in this very post. But it's always worth remembering that actual discussion and disagreement is never a bad thing, as that has a grounded historic context as well; we disagree on forms and disciplines now, just as they disagreed on forms and disciplines back then. The only - often jarring and profound - complication we have is that they were living/dying and breathing/not-breathing it, whereas we are resurrecting the martial art/s and have, in many cases, centuries of lost time to make up for, as well as huge gaps in our historic knowledge due to lack of decent source material across cultures and eras.
Even when applying textbook techniques against other textbook techniques there is a danger of the 'soul' or true purpose of the martial art being lost or corrupted; Matt Easton did a short video a while ago on the prevalence of one particular guard position, cautioning that it is so popular because it works well in modern sparring and bouts, and that such a mentality risks veering away from practicing a historic form. Or, at least understanding and appreciating precisely how that position was used and why back in the day.
Oh, and a last point on the video: ideally he really needs to pull the camera back. If you're going to be discussing technique at all, then he's mostly missing a hugely important feature which shapes all techniques and guard positions; footwork, particularly of the rear.
A successful technique is never just how you hold the weapon, how you cut/thrust, where your centre of mass is, your feet spacing or their angle - it's every element together. To be even more thorough/boring; discussing cuts in a first video about fencing seems premature - everything is built on weight distribution and footwork, so addressing movement without the weapon should ideally be the first topic. Or, show off a little as an opener, and then stress the importance of the foundations. The boring stuff is what makes you move and cut well, or poorly.
Finally, as to the Witcher and fencing? I've not played 3 yet, but I LP'd 1 and played through 2 when it launched. I admire and like the overall authentic-seeming feel and look of its world, and I recall Witcher 1 included a killing blow where Geralt grips the blade and strikes with the pommel/quillons, which - liberties over the animation aside - was the first time I'd seen a sword used in such a way in either a game or film. So they definitely mix in some novel authenticity, with dramatic license.
I'm not that keen on the combos and Geralt's overall style, though.