If that's so, these instances (and who knows which other ones in the world) now host OPs and comments that I as an author decided to delete.
How does one delete information from the fediverse? If this is true, you cannot. Your data will be hosted forever, at least somewhere.
If people use their personal name for an account, this might result in a significant GDPR problem for all instances.
So, now instances are hosting posts and comments that a user deleted on another instance. But that user has no way to 're-delete' the OP and comment.
Now what?
I guess the instances might cross check the existence of OPs and comments on other instances, but the longer this problem exists, the larger the problem.