They are different platforms, so you can’t. If you remove ActivityPub from one Lemmy instance (or server), it would be just one centralized Reddit clone. What makes them special is that every public activity (like, comment, post, etc.) are all sent to other servers in the whitelist. This causes the other instances to also have the same content (copy), making the fediverse possible. Just because you can comment on your instance and that being displayed on some Mastodon instance does not mean you have another account there. Your comment was simply copied to that other Mastodon instance using the ActivityPub protocol.