The platforms (ios, Android, etc.) are very limiting. It is hard to have something always on and listening. Especially apple is aggressive with apps running in the background.
You need constant permissioning and special privileges. The exposed APIs themselves are not enough to build deep and stable integrations to the level of Siri/Google Assistant.
I don't know about this project, but generally when a voice assistant is "always listening" they mean it's sitting in a low power state listening for a very specific trigger like "Hey Siri" or "OK Google" and literally nothing else. As much as they would probably like to have it really listening all the time, the technology to have a portable device run actual speech recognition and parsing at all times with useful battery life doesn't really exist yet.
I'm not sure what that "etc" is supposed to mean, but GNU/Linux phones run desktop OS and impose no artificial limits on you.