I don't what's the motivation for making it iOS only. It makes more sense to choose web platform like another of their products learnsynth (https://learningsynths.ableton.com/).
Maybe they could target the web, but it's pretty clear that iOS is a superior platform to the web for music production apps as well.
Then on top that you’d have the complexity of shipping a cross-platform app and either building large parts of it twice or using something like Flutter or React Native.
It must be annoying for Android users but the reality is that shipping an audio app for Android is (or was a couple of years ago at least) a huge challenge and probably much harder to make a compelling business case for.
WebAudio has similar issues in terms of wildly varying performance across devices (it used to be pretty poor on iOS for example, though I think this has improved) and relative to native, you’re pretty limited in terms of processing power. Again this is improving all the time, but I’d guess Note has a cut down version of Ableton’s synth engines and effects running rather than remaking them for the app, and running something of this complexity with WebAudio is likely to be very challenging if not impossible, especially on mobile.
Android notably does have some good apps for making music on it. There's a version of Cubasis on Android, Koala Sampler is on Android, Google had Teenage Engineering build a music app for Pixels, and I'm sure there's many other apps that have found a way to make it work, though I know for a fact that Koala Sampler has had issues with input latency due to the audio stack on certain Android Devices (where it becomes unplayable).
Apple's done a lot of work to make it a viable workflow on iOS/iPadOS. inter-app audio, AUv3, actual support for tablets, and a good audio stack are just some of the things they've done to make it a real platform for musicians. Google would have to put in a lot of work to make Android at all attractive to musicians and frankly I don't even know if it's worth their time. Still, they can do it and they've done a fairly good job of improving the state of Android for visual arts on tablets, so it's not impossible. Just a lot harder.
Google did introduce Oboe a couple of years ago which was a new, much improved audio API, so they have put in work there. But yeah, there’s nothing like AUv3 as far as I know and the whole ecosystem must be so far behind what exists on iOS that it would be hard to justify many devs putting in the work. But it is possible to ship cool stuff with work for sure!
And you are right iOS is still the worst in Web audio. But if I'm to make a choice, I choose Web audio for the cross platform support.