Personally, I have ONLY primed on a Dillon 550 whether it be progressively or 1 at a time single stage style when loading rifle stuff.
Have only set the primer to the bottom of the pocket fully-----never having to "flatten" a primer.
You even quoted
@Walkalong :
...and nowhere does his statement say to "flatten" any primer, just seat it fully so the cup "swallows" the anvil legs.......
I think there is waaaaaaay to much thinking here.............................
THIS IS ONLY MY OPINION...............................squish 'em like a pancake it that's what works for you..........................