he is just making a big authoritative sounding thing based on his personal experience on horses and guessing and extrapolation and some light confirmation-bias research

fwiw Lloyd is very well known for mostly doing this. A lot of his content is actually relatively shallow, or has important shortcomings if not outright errors (though those tend to be rarer) that someone with more knowledge would immediately pick up on. He brings an interesting perspective where he bases things on his personal experience, which is often indirectly related to what he's talking about. Like re-enactment as a way to talk about HEMA or historical warfare. It's interesting and worthwhile, but should not be considered anywhere close to authoritative. Still, he's generally better than most HEMA-adjacent but not-actually-HEMA creators.

However, the video posted by @Dasus@lemmy.world is not quite as bad as you and @PugJesus@lemmy.world might think based on its title. For starters, he's very clear in that video that he's only talking about cavalry, not horses ridden for other reasons, and certainly not horse-drawn carts. Heck, he even specifies that he's not talking about chariots, which were the main way horses were used in battle for thousands of years.

It's actually just a clickbaity title for why the invention of the four-pommelled saddle or stirrup was necessary for cavalry (defined as people who sit on the back of a horse and fight in battles from horseback) to work.

If you want some genuinely bad horse content from Lloyd, try this one where he interviews a re-enactor and modern horse-trainer who claims mediaeval cavalry could be trained to charge into an enemy line...but they'd only do it once. Though even that isn't necessarily as bad as some critiques have made out, because the re-enactor talks about how the goal is to get the enemy line to break, and the "only do it once" claim seems to be if the infantry line holds formation. For example, this Reddit comment seems like it may debunk the claim made by the guy Lloyd is interviewing, but only really in its antipenultimate paragraph, and even then it's hard to draw conclusions on how strongly it refutes the claims in the video (Elandslaagte is described in Wikipedia as a cavalry charge that began after the Boers were already retreating, some of the other battles are described in that Reddit comment as involving the cavalry charge breaking the infantry, etc.). On balance it seems likely the claim in the video was made overly-strongly, but I think it probably isn't quite as terrible as it may seem on the face.