As you have not granted mastodon.social rights over your content by signing up to them, they do not have a license to redistribute your content - in fact you could sue them to the ground with GDPR if they would refuse to delete content (which is the main difference - revocability), and the terms of service of the server you are on dictate what they can or cannot do. In fact technically speaking it's questionable whether mastodon.social is legally allowed to redistribute your content as the user of another instance to their own users, but that is a separate issue entirely
You are misinterpreting the issue entirely, not only Mastodon's stance but also how IP law and the fediverse work
Obviously, I am not a lawyer, but I am an open source dev that handled IP things on a best effort basis.