Interoperability is what was promised and what we’re hoping for. It’s implied in there being a standard at all. If devices follow the standard (which they have to do to use the Auracast name) they should work together. Many people will add “in theory”. Is it possible for a manufacturer to add features outside of the standard and wall them off from the competition? It’s too early to say.
What they possibly can do is give their own devices preferential treatment in their apps. The Resound and Jabra apps might be examples. There are two tabs within the app to control the interaction between up to two streamers and the hearing aids. You can control the volume and mix between the streamed audio and ambient sounds- probably other settings as well. Presumably things wouldn’t be as slick with a third party transmitter. The app is used to control the aids so, for most users, it’s always to hand. A big advantage for their own transmitters over the others.