Yeah... the key word there is "assume." That's a handwave to EXPLAIN the error.
Like in the X-men films, when we just assume that Alex Summers' parents were ridiculously young when he was born, which would explain why they are still middle aged and raising his 16 year old brother in the mid 1980s (and Alex, who was a young adult in 1963, is technically old enough to be his father). That, plus Moira not remembering that she met Charles Xavier BEFORE they left the CIA facility to go to his school, is a continuity error that can be handwaved with fridge logic.
A retcon in this example would be Mystique being an old friend and former roommate of Professor X. It's not like they ever said or did anything that would explicitly rule it out, but slipping it into the prequel films (and then adding the "you age really slowly!" lines from beast) changes her entire backstory.
I use these examples because the X-men films were written by monkeys and SUCK at continuity.
Right. That's a handwave to explain a continuity error. When they actually FILM it, it's a retcon.