Not only is it not Linux, but the operating system is closed source: https://en.m.wikipedia.org/wiki/HarmonyOS. According to the article it’s built on top of “OpenHarmony,” but that does little to assuage my disappointment.

Yes, in that it uses the Linux Kernel. It’s not typically considered a Linux distribution because most everything else in the OS is custom and Google-suffused. In particular Android lacks GNU libraries and tools.