I don't even think they have to do that. I think the law will just mean that they have to allow other apps to implement their protocol, not forcing them to follow a specific standard. Like, they can still use iMessage and add new features, as long as there's some way for third party apps to use it as well.
Idk though maybe I misunderstood it
No, I don't think so. You can set a standard that defines a baseline of features that all interoperable messaging apps must support. But that doesn't preclude apps from offering additional functionality over and above the standard.