Google’s implementation is just a bunch of weird text sent over texts. It’s using the standard as a transport for their own extensions.

That doesn’t mean other apps can’t communicate with Google’s client. You can’t use Google’s special sauce like encryption (without reverse engineering the details) but you could very well use your own.

What developers want when they say they want Google to open RCS is for Google to take the RCS code out of their messages app and put it into a standard API, the way SMS and MMS work, so developers don’t need to go through the trouble of implementing RCS and basically be able to use the existing code without changes. That’s a nice idea, but I don’t think it’ll happen any day soon. Google generally doesn’t contribute their closed source stuff back into open source Android. There is one type of RCS authentication that requires interacting with the SIM card, which only privileged (system) apps can do. All other forms of authentication can be implemented by any app. With multiple SIMs, you could even use different apps for different services. However, they’d need to actually implement the RCS spec, which everyone is hoping to avoid.

iPhones will be able to message Android phones without Google lifting a finger. Flipphones will be able to do it too. Things like video calling, sending money, and stickers can all work, because that stuff works over standard RCS. The lack of encryption is a pain (though I doubt it’d take long for someone to reverse that) but it’s not something existing cross platform texting solutions offer.

Google Messages is the Microsoft Wordpad of text editing. Pieces of it are open, most of it is composed of public APIs, and it’s no more than a very specific implementation of existing standards.