I had been a bit puzzled about this prescription in view of all the low-ish rep work. Then I started to structure my warmup a bit more and things started to make more sense.
For the Scap pushups (sPL/PE1), 21-15-9 seemed a reasonable prescription, and I tried it out. It felt great, and for a while I was thinking "I should do some math to scale those numbers down for the lower rep exercises" - until I thought a bit harder and realized there was no need for a calculator:
21-15-9 = 7*n - 5*n - 3*n, and the middle number is the number of reps/secs for mastery - so there is your template...
What confused me at first was the "21", which is not a number that you find in the mastery requirements. But if you have mastered an element at 5 sets of 5*n (n=1,2,3...), doing 7*n reps once as a first set surely is possible.
So with this in mind, the warmup template can be applied to anything in Foundation. For example:
Mastery of 5x10 r PPP became 14-10-6 r
and the 5x5 r/side Xiaopengs as iM became 7-5-3 r (or 7-6-5 because I love them so much)
and so on...