Surely the implication is that when you 'burn whatever is behind that UserID' you would delete the records of who DeletedUser4510 was friends with (and who was friends with DeletedUser4510).
If all you do is rename someone to DeletedUser4510, you pretty obviously haven't deleted all the data you hold on them.
> you would delete the records of who DeletedUser4510 was friends with
You erase the friend-linkages...
... Then you find that Jane Doe made a post in response to a blanked DeletedUser4510 post, and responses can only be made by friends, so therefore Jane was at some point friends with DeletedUser4510.
So you put in tombstones for all posts and all post-to-post causation links.
... Then you find that the entire Doe family is tagged in a photo by Jebediah Doe, except for one guy, and the comments are every family member and somebody named DeletedUser4510...
Anyway, my point is that it's really easy to get caught in such a fog of relational data that gaps merely change a certainty of identification down to an extremely high probability, and an event-sourced system -- by design -- makes it extra difficult to remove data or to even plan for its removal.
If all you do is rename someone to DeletedUser4510, you pretty obviously haven't deleted all the data you hold on them.