I don't pretend to understand the "keyfile" thing here. I guess that's what I'm asking: if this is at bottom just a symmetric key cryptosystem, like, even an encrypted DMG is "safe" in that sense.
A big reason to mention quantum safety is just to make it an explicit goal. If I've accidentally chosen something that even smells uncertain, it's a bug. There are plenty of ways for symmetric key cryptosystems to accidentally weaken themselves, like leveraging a 128-bit intermediate key at some stage of the process.
Now, that might not even weaken things realistically, for all sorts of reasons. But I want something that you look at and say "yeah that's obviously fine," without having to think much. No surprises.