It's a kernel per se, and any OS built on it is considered a Linux OS. "The Linux kernel is the core of any Linux operating system." - kernel readme
Linux.com refers to the kernel itself as an OS though, I think to relate to people who have only heard of OSes like Windows. And calls Android a platform based on the Linux OS :S
Pity that Android doesn't run any Linux userspace application, unless one makes use of workarounds, like termux having to fit within the constrains of Android sandbox and NDK limitations.
Well they've contributed upstream to Linux kinda as a result, so that's nice, but Android being Linux is no reason for me to want to use it. Like, so is the Amazon Firestick.