No. They can’t sue you for asserting that you libel’d them because you are not causing harm to their reputation. You can make assertions about anybody on the internet, but they have to prove they were harmed reputation ally by what you said and they would lose because they don’t have a reputation attached to who they actually are in real life associated with their Lemmy account. They’re full of shit and this is an attempt to strong arm you.

Report them and block them and be prepared to block and report any new account a harassing you until they get themselves an IP ban.