The problem is, its usually cheaper the more things you can shove into the 1 hardware item, so you have your cellular hardware in the same chip as your CPU and GPU. Not much a ROM can do about this unless the chip itself supports disabling direct memory across the two items, + does it correctly, + doesn't allow it to be reversed from the other side, + you would also need the datasheet to find out how to implement this.

Generally why privacy roms don't support more than 1 or 2 brands total, I guess.

There are also platforms with strict division between the seperate parts of hardware, la pinephone and the librem5