I'm surprised so much has been written on this topic. I'm probably naive in thinking this is overcomplicating things. If you don't control the url to your content, you control nothing.
I think you're right in general, strictly and technically speaking. But with that mindset, we basically cannot have any control on the internet unless you own your own IPs and a ASN, but even that is "leased" or ultimately controlled by someone else, it can always be taken away from you, one way or another.
So instead the discussion becomes about "degrees of control" if you will. Using Facebook to host whatever would be on the lower scale of "control", having your own website at some hosted service with possible exit would give you a bit more control and having your own domain pointing to a home server would give you even more control, and so on. Same goes for what "decentralized" really means, it's more of a range than a boolean of "yes/no decentralized".
Rather than this black and white view of "You control nothing unless you control the URI", (ATProto at least) people are trying to figure out a balance between "You control everything" and "It's a huge hassle to manage everything yourself".
These are used throughout the system, for example if you look at the output of a feed generator, its just a list of these URIs