They wrote it from scratch in C++ so they could avoid some of the legacy cruft in Android. And they are getting adoption. It's a major OS in China and in many developing countries (phones with it are cheaper, and it flies on underpowered hardware!)
Before we judge the magnitude of this event (HarmonyOS existing and being successful), let's remember that last time anyone tried to disrupt the duopoly Android-iOS, it was MS, the largest company on earth by market capitalization at the time. And they failed.
Well, it very much looks like Huawei is not failing. We in the west don't see it as much, because propaganda is working well. But last tech conference I attended (GITEX Berlin, if you are wondering), had their app available to download with... 3 logos, not 2. Harmony OS was there. This is a major win for consumers all over the world.
And this being HN, I hope the inevitable comment "but China!" is slightly more informed that the average internet user.