Google likes Android ROMs because they pacify the developer community from working on real competitors, while not presenting any meaningful threat to their control of the majority of Android devices. The MADA that prevented OEMs from shipping AOSP is probably dead but what hardware manufacturer is going to risk Google's ire by shipping something.
As it stands, and the way things are devoloping, accurate. But as the relevant systems are an integration of hard- and software, significant work needs to be done on the former as well. And I've yet to come across a Linux phone (or phone-like pocket computer) that ticks most of the neccessary boxes.