Probably because that post they linked was posted by their lemm.ee account, not their piefed.social account. The problem is that lemm.ee shut down; that Lemmy instance no longer exists. That post had federated to piefed.social at some point before lemm.ee shut down, so that is why it shows up there. But that post had not federated to lemmy.zip before then, so you are not able to find it by looking it up there now.
Probably because that post they linked was posted by their lemm.ee account, not their piefed.social account. The problem is that lemm.ee shut down; that Lemmy instance no longer exists. That post had federated to piefed.social at some point before lemm.ee shut down, so that is why it shows up there. But that post had not federated to lemmy.zip before then, so you are not able to find it by looking it up there now.
Ah, of course. Thank you!