"You may ignore #1 & #2 if you strictly observe #3."
Understand that I make no recommendation to anyone, we all select our methods and live with the results don't we?
That said, I've broken all three of those rules for a lot of years and will continue to do so, those split collets are too clumsy to use. (And I see no difference in how my puller works with either the collet or a shell holder.) There is a lot more about that than we've been told, IMHO. Meaning, I seriously doubt the story as it was told.
Yeah, I read the original post and saw the photos that was said to show what happened; it did go off but it's not plausable - to me - that a shell holder could possibly cause a primer to detonate in an impact puller! So, why it happened I don't know but but I just don't believe the use of a shell holder, per se, had anything to do with it. And I really doubt that the end results would have been much different if that dude had been using the three piece collet device.