Wow, I see Lars Anderson has progressed even further into the absurd! Amazing feats.
So I actually got a lesson into his basic technique last year. When the video talks about shooting from the right side of the bow and not the left, what's really going on is a switch from a finger release (traditional in much of Europe including Britain) to a thumb release (traditional in many other parts of the world). The finger release is not well suited to holding your arrows (because three of your fingers are engaged leaving on the thumb and pinky free), whereas it can work in the thumb release (optimally only your thumb and forefinger are engaged, leaving three free fingers). I can not do it fast, but starting with four arrows in hand and moving what feels slowly and deliberately is on par with my speed-shooting finger release speeds. I've tried seven arrows as well, and ten is the same principle. After that you run out of space between your fingers. The person who showed me said that 7 or 10 arrows slows him down a bit, but we're talking fractions of seconds here.
It does take some time to "load your hand" when you do this. When Lars is grabbing arrows and shooting, at most it's 2-3 arrows at a time. I don't think you could do the max speed 10-arrow trick and quickly get back to it. It's something you could start a battle ready for, but then when you exhaust those 10, your speed would drop. Obviously if you are trained to absurd levels, it doesn't drop as much as for the rest of us.
My problem is that my accuracy is terrible with a thumb release. Need more practice! And my thumb doesn't tolerate it very well. But those aren't problems for people who have trained that way rather than finger release. As I noted, in many parts of the world it is standard. In some areas where finger release is standard, it quite possibly became that way because modern archery in many areas was essentially re-introduced as a sport starting in England, so they wrote the books and trained the teachers. Thumb release may have been even more common historically.
The English longbow is a bit of an outlier among combat bows. The typical bow used for war was 50-60 lbs draw everywhere else, and 90+ for longbows. (Though in fairness, note that a recurve bow has more power than a longbow of equal draw weight, and recurves were common in many arrows, so the functional gap is probably less than that makes it sound.) Although in the video he says his technique works with heavy bows, the one he is using does not look like a 90 lbs bow to me. Is finger release used with longbows by chance or because it's needed for strength? I'm not sure. I've never drawn a 90 lbs bow either way....