I actually like that video, but here's two observations: 1) some HEMA enthusiast speculation / reenactment techniques are not necessarily always the gold standard for understanding how these weapons were utilized in actual warfare, and 2) even if we credit this a reasonable summary of how to utilize the shield with a sword (and at a minimum it at least gives us a decent indication of the speed of the movements someone with the right conditioning can achieve with weapons of presumably more or less period appropriate weight) the video doesn't really support your position to the extent you suggest: much of it seems to presume very little lateral movement and there's no reason to believe they aren't just sharing established doctrine for how to strike while in the shield formation or similar close quarters fighting, but they are merely demonstrating these strikes one on one because of course that's the only way to do it for a youtube video demonstration.
[Edit: Also, not for nothing, but the video demonstrates an approximation of real world fighting between two goes with more or less identical weapons and similar proportions and normal rules of physiology and physics: all of those things would go out the window if we're talking about how they might try to render this all of this in practice for the game.]
And look, I'm sympathetic to your credulity here, and of course I can't say for a certainty that no historical warrior would opt to choose the security of a shield, or even that for some of them it might not turn out to be the right decisions, but there are some factors you don't seem to be taking account of (or at least are not appreciating the full impact of) which significantly limit the feasibility of that strategy when utilizing the heaviest of shields. Again my experience does not involve shields, but as a general matter in weapons training, I can tell you that defense is more exhausting than you might think--in fact, if your opponent is setting the pace of the fight and your armaments (whether it is your weaponry, your shield, or even your protective gear) are substantially heavier, it can actually be more fatiguing to be on the defense than on the offense--you're actually bruning more calories and building up more lactic acid and strain in your muscles over a longer period.
If you had two theoretical trained period warriors, of equal skill and experience, and one is lugging around a heavier variant of the scutum (note, these would be heavier than those in the video you posted), then the other combatant, provided enough room to maneuver, is just going to make the other guy chase him around. And if scutum guy's only answer to that is to try to stab over the top of the shield on offense and angle said shield to deflect blows on defense, he's not going to have much luck doing anything other than wasting his stamina while lifting that shield (which blocks his ability to attack almost as thoroughly as it does his opponent's at any given instant) to absorb blows. Meanwhile, his own strikes are far from ideal, coming from a severely restricted range of motion, and the shieldless fellow can still parry pretty effectively. Scutum guy can use the shield itself to feint and create distance, maybe even glance his opponent's weapon wide and have enough time to strike, but trying to find these opportunities would, again, quickly prove exhausting.
Look, I'm not going to pretend to have the confidence that I can 100% predict who would come out on top at the end of that fight--I just don't know these weapons well enough to have that level of confidence. And that video has given me pause for thought by demonstrating the strikes where the shield is flipped along the transverse plane over the incoming blade (I find that a very nifty maneuver). But I can tell you what my instincts say about who I think I'd like to be in that scenario, if we were talking about a life or death struggle. And those instincts are informed by some significant practical knowledge, not whatever Orlando Bloom might be doing in some awful film. Am I potentially biased by a pressure to seek out what is a little more familiar to me? Maybe, but let me put it this way: my main thing is tonfa, which weigh waaaaay less than a steel tower shield and gladius, and yet it's still exhausting to be on the defense. Even if you rarely spar and are mostly just drilling, trust me when I say that absorbing impacts through your forearms is a tiring exercise.
Anyway, that's my hot take. Wow, we really got off an quite a tangent here and away from the original topic of how feasible it might be to simulate the style in Soulcalibur terms. Not that I'm complaining. Even with the 'hollywood' snark, I'm enjoying the discussion: at a time like this I could use a little contact sport, but of course that's about at the tippity top of stupid things one could do right now :[