I think parent was referring to how challenging it is to move data (files like photos and other types of files, all of which are only accessible through apps with those specific capabilities) out of the Apple mobile ecosystem and to something non-Apple-ish.

This is still true even if you use a Mac as an intermediary (if you have one), which also implies that you're probably going to be using iCloud to sync those as well.

Bottom line: it's exceptionally difficult, even for tech-forward Apple-philes, to move your own data off your iPhone without actually going DEEPER into the Apple ecosystem, and Apple has been actively removing capabilities and neutering apps like NextCloud etc (always for 'privacy' or 'security' reasons) to make it MORE difficult to exfil your own data.