Well, What he says makes sense. Think of it this way.
If more muscle fibers were recruited as your repetitions increase then the weight lifted should get easier as the repetitions raise. Meaning, Your 10RM should be higher then your 1RM. However, this isn't the case... Simply speaking, The muscle fibers recruited, adjust to weight being lifted (regardless of speed of movement) but movement itself can be the deciding factor aswell.
By "very" large motor units he is specifically talking about Type 2b muscle fibers. Type 2b muscle fibers contract the fastest While type 1 contract the slowest (slow twitch MF). Of course I'm neglecting to mention the "in-between" fibers 2 and 2a.
Now, He is stating (without directly stating it) that once your lifts begin to slow. You are no longer using type 2b(and other type2MF). You are using type1, the slow twitch. Now, Physiologically speaking. Type 2(&a/b) are naturally "larger" then type 1. Meaning if hypertrophy is the main goal. Exercising past the point which type 2/a/b are no longer being used. Is of little use.
Now, This is not new. If one studies muscle fiber's in general. They would understand this simple principle.
For the record, I am not a human anatomy expert here so take what I say with a grain of salt here...