My understanding is that Apple will let you collaborate after what you're working on is public. They just don't want people accidentally sharing unreleased stuff with people who aren't cleared to know about it.
I've been told by folks who work on infra there that they need to file requests to even talk to the previous owners of code if that person has moved to a diff role. So I don't think it's just for unannounced products.
Once something is released, there's general access given for that particular software/hw but it still needs to be requested. In a weird way, it creates some internal excitement on "secret" releases but obviously makes development much more difficult.