So the only things you secure are access to existing posts,

The only thing I secure is that for a given Action by a given Actor it can be validated that those were signed with a given key.

So if [email protected] creates 10 posts, 100 comments and is the moderator of 5 communities (on other instances) then the public key would secure that only that user can interact with that data.

Everyone can interact with that data, but since those are signed with a specific key the sign would become invalidated.

Now that user wants to moderate one of those communities again, are they treated like a different user (because keys don’t match)?

Since the key and signature are just additional attributes of the Actor object they’ll be the same user federation-wise. An instance admin needs to manually validate why the Actor uses a different key now. If the Actor is used to perform malicious things it can be verified that those things are done with a different key. What’s done with this information is up to the instance admins.

If they are treated as separate users by other instances, they could just write another legitimate moderator

Exactly. Key signing does not prevent social engineering.