Sure, I'm not saying it's a good product. But this "criticism" reeks of amateur-level understanding of Android and doesn't seem valid. Criticize that it's overpriced and useless - but the way they built the software makes sense.
You can't because the standard Android phone operating systems and Play store restrictions don't allow it. You need to build (slightly) customized AOSP and flash it to do the same thing. Or root the phone, but that's still very distant from "it could be just an app".
BTW I would love if there was a way to do it. I have my own humble voice assistant system and just installing it as an app on my phone to fully replace Google Assistant would be perfect. But you can't do that (and of course it's a total nonstarter on iOS), so it runs on a Raspberry Pi with standard Raspbian and the app package - practically the same solution.