Why would someone think that?

Because the post I replied to, the actual thing I am addressing, makes an assertion that isn't entirely true and could lead someone uninformed into believing they can have their information removed platform wide.

What is the difference?

Not everyone is concerned with someone digging up dirt or wildly compromising material. Most people aren't special enough to be worried about that.

Most archives won't be globally search indexed. An archive won't show up on a federated search. There is more legitimacy to a federated version over someone reposting a screenshot (at least in perception, how federated could be altered or forged is another topic).

I also mention there are other reasons one might want to remove content. Just look at reddit right now, some may simply want to revoke support for a platform sometime in the future.

Sure, there could be a future where this is addressed. It isn't right now.

I don't disagree with you in the larger discussion on persistence of data, and I'm not shitting on the platform. I am adding context to a scoped subtopic of it.