I can't say for sure. They may be tied with certain changes to PGML.pl that made it easier to use PopUp questions in PGML in the first place. And those changes to PGML.pl may have been concurrent with some lower-level things too.
2014 and 2015 saw a lot of edits to parserPopUp.pl:
and if any of those are tied to other developments, it could be trouble to just bring in the new version without a full upgrade.