I didn’t know how much work they put into customizing it, but being derived from Android does not mean it isn’t custom. Ubuntu is derived from Debian, that doesn’t mean that it isn’t a custom OS. The fact that you can run the apk on other Android devices isn’t a gotcha. You can run Ubuntu .deb files on other Debian distros too. An OS is more of a curated collection of tools, you should not be going out of your way to make applications for a derivative os incompatible with other OSes derived from the same base distro.