Yeah in truth I haven’t used physical money in ages either. Tap to pay is so much more convenient, and even before cards started having that, inserting and entering your PIN was still way more convenient than the annoyance of cash. If I find out a place doesn’t take card, I’ll usually just walk out.
Piefed is compatible with Lemmy in the sense that a Piefed server can talk to a Lemmy server using ActivityPub, but they are different software beyond that, and so they have different APIs at the user level.
It’s not really different from how Mastodon is different software, even though Mastodon users can see and reply to Lemmy posts.