@kegill at present, they are not. The AT Protocol is different than ActivityPub, which is close to SMTP in ways; there are efforts to bridge them, but they are controversial in some quarters: https://techcrunch.com/2024/02/14/bluesky-and-mastodon-users-are-having-a-fight-that-could-shape-the-next-generation-of-social-media/
@mmasnick listed reasons why the AT Protocol may have advantages.