They would work in parallel. If the GC wouldn’t finish when the trash-filewalker removes pieces, it would be captured in the next run (it’s running every hour by default).
However, not necessarily in the time order, see
But the amount of the deleted data will be the same. I do not understand the author’s concerns, because it’s related to the speed of deletion, and the sort order doesn’t affect the amount of the deleted data. The order also doesn’t makes any sense, because the trash data will be deleted sooner or later, if it’s expired.