PieFed absolutely has a post scheduler, which I use all the time.

By the same token, OP’s posted ~1100x across 11mos on their Lemmy acct, so it seems quite possible they’re using some kind of Lemmy scheduler. Since you say it’s not in Lemmy proper yet, I’m guessing they might be using one of these:

https://github.com/RikudouSage/LemmySchedule
https://github.com/stevenvergenz/lemmy-scheduled-post-bot