They probably don’t see Proton on any hardware as up to the standard they have for stability and consistent performance across the board, I imagine if it is opened up to anything there will be a lot more edge cases causing problems. I imagine Proton will reach near 100% rate for supported games before they would release it to any hardware, basically when it makes sense to pivot Proton development to deal with edge cases rather than expanding its support, if that all makes sense.