It should be a signature that is sent together with the ActivityPub Object. Yes, if the signature doesn't match, the content, whether a post, comment, favorite, upvote, etc... should be dropped.

Here is the source code of the library that lemmy uses to handle incoming objects and you can see that it does a call to verify the signature of the actor:

https://docs.rs/activitypub_federation/latest/src/activitypub_federation/actix_web/inbox.rs.html#18-54