I hear or read this sentence from time to time. But I have to admit, that I don't understand it.
Because in the practice I know, one of different ways of keiko is to resist, to block, to try to stopp the technique of tori.
You're describing henka waza.
What Katherine meant, I believe, is literally what she said. Look at it carefully. The technique, one just demonstrated by instructor, the one everyone is practicing repeatedly, CAN be stopped by a wicked-minded partner.
With henka waza, they don't know what's coming next, which includes atemi, and it's harder for them to stop it. Hence again, Katherine's statement about them KNOWING what's coming, and KNOWING they won't get hit.
But regular training method at my (former) dojo doesn't include henka waza. At all. It's considered "dirty" I guess.
Therefore, you're railroaded into that one specific technique, the uke knows exactly what's going on, and if the uke abandons the concept of sincere attack/mind, of simulating that specific energy, one out of many, you're never going to pull it off.
When the uke's goal is not to attack, but just to withdraw/make YOU uke/block a specific set of movement/energy, their behavior is no longer even remotely natural or realistic, and this entire interaction becomes a useless dud, an artificial construct existing only inside dojo bubble, teaching nothing of use.