That’s exactly what the GSMA is saying. Universal interoperability means not having to use Google’s encryption method, therefore iPhones and all other phones including customized Roms should work.
Now, they could absolutely get in the way of this, but I would hope that they would think it’s not worth the effort.
You’re correct, but that’s not really the point of the origin question. Yes they always could, but why are they doing it now? Why this uptick in meddling?