if ($ttl && $ttl < PhabricatorTime::getNow()) is logically equivalent, I just find the separate clauses a little easier to read here. (I think I usually prefer separate clauses unless there's some reason they're worse, and usually find if (...) statements with exactly one test easiest to read, although this is super subjective and I'm probably not consistent about it.)

I'm ignoring only expired temporary files (instead of all temporary files) because it's possible for a temporary file to expire a year from now, or a hundred years from now, and effectively be a permanent file. In particular, file.allocate lets you specify any TTL epoch with deleteAfterEpoch, and the lower-level APIs let you pick any ttl.relative or ttl.absolute. We could just skip these and effectively say "exporting data deletes your /tmp directory", but that's a slightly stronger statement than "exporting data doesn't bring any files which were recently in your /tmp directory but have already been deleted". Practically, I believe the upstream only ever uses a 24-hour TTL on anything right now, but file.allocate is an outward-facing API so any install could theoretically have long-lifetime temporary files.

(Actually, it looks like meme-generator output images have a 30-day TTL.)

Maybe another vague argument in favor of handling temporary files is that bin/files migrate also isn't only for exports, and I could imagine wanting to test the other options on a temporary file.