The difference is that the HarmonyOS API was just a fork of Android, even if the underlying system changed (and it wasn't changed that much). Fuchsia is completely different, from the kernel all the way to the API.
The end result isn't what's ambitious. Linux can do this today, and run Windows applications at the same time. What's ambitious is getting different APIs to work together well on a system that's designed radically differently. The overlap lies in Flutter (the "we'll ship our own GUI toolkit with the application code" approach), not in the underlying framework.
I imagine Apple could quite easily get Android apps to work on iOS if it wanted to. They need a wrapper for Linux calls, which they can borrow from Google in this case, and they need part of the Android framework, which can be made to run on the wrapper. It wouldn't take more than a year for the first proof of concepts to come out, I'd reckon. Apple just has no reason to want to run Android apps because they're not the ones that are in desperate need of market share like HarmonyOS.
The end result isn't what's ambitious. Linux can do this today, and run Windows applications at the same time. What's ambitious is getting different APIs to work together well on a system that's designed radically differently. The overlap lies in Flutter (the "we'll ship our own GUI toolkit with the application code" approach), not in the underlying framework.
I imagine Apple could quite easily get Android apps to work on iOS if it wanted to. They need a wrapper for Linux calls, which they can borrow from Google in this case, and they need part of the Android framework, which can be made to run on the wrapper. It wouldn't take more than a year for the first proof of concepts to come out, I'd reckon. Apple just has no reason to want to run Android apps because they're not the ones that are in desperate need of market share like HarmonyOS.