I was curious about this too, but digging around on the internet doesn’t seem to give a definitive answer to this question. The “breaking Android application compatibility” story is real, see this Technode article.
What I think seems to be happening is that Huawei is developing HarmonyOS the way GNU/Linux came out of Unix, replacing bits and pieces at a time. They started out using many prominent Android components which led to some commentators dismissing it as just an AOSP fork, but over time they’re diverging into a genuine third mobile operating system, including their own ABI and development toolchain.