It's been a while since I posted anything...
There seems to be a couple of different things going on here.
With regard to ukemi. For me, uke should respond to nage appropriately for the situation. During kata, uke should be an element of the kata - it's funny, but when we talk about kata, we will chastise uke for not moving right, but we forget that nage needs to move right, too. Nage is responsible for correct kata, too. During fundamentals, uke should should respond to nage to provide feedback. During sparring, uke can be adversarial. Just make sure expectations meet actions.
With regard to internals and slack...
O Sensei's famous "masakatsu agatsu" comment can be interpreted as a comment about balance within the body by removing slack. Something like, "stand without slack," is a translation of that kanji. One can look at the source and see the importance of that comment. I think its also important that the comment was not about removing the slack from some else.
No comment on slack but Jon - in sparing there is no uke (or any other defined role) and I really do think that it is a rare instructor that emphasizes the correction of uke over nage. Perhaps I lost your point, I did get confused.
Still kata really is a balance between uke and nage and in the former case, at its most basic, we don't want uke moving ahead of nage's action (i.e. throwing themselves). At higher levels uke supplies different levels of resistance AND (to the point of those videos) other actions which provide feedback to nage.
I am still trying to get my head around what those actions in the video represent. The answers so far have been disappointing.