Not to put words in Jim’s mouth, but I think his response comes from how “finishers” usually seem to imply that you save something that is going to kill you for the last exercise, but assistance shouldn’t really be treated as such. It’s assistance; not conditioning, “Don’t major in the minors”, etc. Here’s an example that Jim himself said about kroc rows (Proper Programming Question - #6 by lukets72)
Kroc rows to many can be seen as a “finisher” because you are supposed to not be able to do another rep, but that’s the wrong way to look at it. Notice that Jim didn’t say “Kroc rows will wear you out so save that for last” but instead recommended that you do another exercise with VERY light weight, and that is the point of his 5th principle in his new book. It’s about balance and what you’re doing. Perhaps “finisher” seems to imply that you aren’t taking balance into account. A person who does “finishers” might say “Okay kroc rows will prevent me from doing 225x10 barbell rows so I’ll leave the kroc rows for last” and that doesn’t take balance into account. Assistance is assistance and no more. The main lifts and supplemental work is your chance to push yourself, assuming that’s what the template requires you to do.