An instance the size of lemmy.world I would hope has separate read and write databases and the delay would be updating the read database after the write db has finished the transaction.
Even though they are strictly local the nicks are still saved on other instances.. And that's what's going to take time If he's right.
Down vote me all you want but that is my logic. Would be great to hear from some of the devs on how this actually work.
OP Lemmy is build on desentralised servers. There is not just one database but potentially 100s of them that has the info that needs to be deleted. That's why it takes time. I do get the frustration if you don't get this and Lemmy could be better at explaining this but that's how it is. Have patience.