Given non-Catholics don’t believe in popes, why would this be surprising to you?
Not surprising, but interesting...
Given that the concept of "Divine Law" is a revelation (protestant term) as defined by Catholics, revelation should be based on and inspired by Holy Scripture...
As far as I can tell the sole requirement to be leader in a body of christian believers is to be a Christian first...not a Roman Catholic... (Not completely sure Pope Frankie can pass that test) ..
There is nothing in Scripture that makes non Catholic believers apostate, heretics and schismatics...
That's strictly is man's tradition... not revelation, thus not Divine Law...
Given Catholics didn’t manage to have any 'scripture' about popes to place into the Book it compiled; why would this be surprising to you?