The top of the article addresses this a bit: yes, Android in theory is AOSP [Android Open Source Project] which is still Linux based. (For now, lots of rumors every Android release that a kernel switch is coming.) But AOSP has lost a lot of power as Android moved so many things to Google Play Services [closed source], which leaves room for Linux-based phone OSes that disown/disavow AOSP entirely because what's the point of playing ball with a half-closed ecosystem anyway? (The article discusses the Ubuntu Touch fork still seeing some maintenance despite Canonical giving up and decides on Sailfish OS.)