Rick, I'l have a try, but I may get corrected here
My impression of each is pretty much in line with the way you described it. A knockdown might more often than not, happen while going to weather where there is a very strong puff or gust, and it comes from further aft, so that you cannot respond quick enough, and you are simply overpowered, knocking you almost flat. On a keel boat you may not put put your mast in the water and you may not round up. a knockdown can happen with a spinnaker however, and I believe that is what happened in your first link. They were running with the wind aft with the chute up when they got a powerful wind shift, which in their case went forward, so the main spilled the puff, the chute went wide as on a reach and filled. So with the center of effort forward the boat stayed in balance and just went over.A broach is normally with a spinnaker broad reaching or running, where you get overpowered, increase your angle of heel. get uncontrollable weather helm, and round up while getting knocked flat, with the spinnaker in the water. The mast does not actually have to go in the water. Your second link is pretty close to what I would consider a spinnaker broach. He was beam or close reaching and was rounded up and would have been knocked flat except his crew let the sheet and the guy loose and he recovered without getting knocked flat. He almost got in trouble though by letting the guy loose with the sheet. I have seen a spinnaker in a situation similar to that with figure 8 knots in the end of the guy and sheet, where both hung up and the spinnaker refilled and kept pulling the boat over. If you just loose the sheet, the spinnaker should flag. He recovered though so it wasn't quite a broach (in my opinion). I would call it a round up. Now I'm prepared for the dissenting opinions.Joe S