I’d say think of it like how the same website might be sent to both a modern web browser displaying to a large screen AND to an ancient greyscale cellphone.
ActivityPub takes care of broadcasting content to different applications, but after that it’s up to the application to figure out what in the world to do with it.
Yeah, the ancient cell phone interface might not do too well with a modern webpage, but http leaves that up to the phone to figure out :)