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.)

You can build various, vastly different operating systems around the same kernel. When people refer to "Linux phones", they usually mean the family of systems often referred to as GNU/Linux distributions, which Android very clearly doesn't belong to.