Does this mean third party apps will be able to interact with whatsapp?
only when the service specifically requests it and agrees to Whatsapp’s terms.
So I [in theory, I don’t know how to start with this on a technical level] could make a third-party Signal-compatible app, but allow it to connect to Whatsapp instead of Signal? Even if I can’t use my Signal account to contact Whatsapp people, that’s still potentially useful. Although I imagine the terms I’d have to agree to to do so would be full of nonsense that stops this being remotely feasible.
could make a third-party Signal-compatible app, but allow it to connect to Whatsapp instead of Signal?
you’d have to create a messaging service, not just a client.