ActivityPub is a standard, Lemmy, KBin & Mastodon are open source applications built on the standard. It’s the same relationship as Hypertext Transfer Protocol (HTTP) and Chrome, Safari, Firefox, Apache & IIS.
As a client/server architecture, Lemmy is no more or less vulnerable to malicious actors than a web browser or a web server. You’re at least as likely to have a rogue admin mishandle data as someone build Evil-Lemmy. While I consider myself a good netizen, if you delete this post right now I’m still going to have a copy for at least six months because that’s my current backup retention for this instance.
I’m no GDPR expert but I can’t see how an instance owner who does comply with GDPR can be punished for instances they don’t control not deleting federated data. There are ongoing conversations throughout the Fediverse on this topic.