And that seems more like a marketing issue, under the hood there actually are differences between macOS and iOS. Catalyst hopes to fix this but it's still pretty rocky imho. Google also did something similar with running Android apps on chromeOS.
iPadOS, iOS and watchOS would be the same for Android phones, tablets and watches. So it doesn't seem like a fair comparison here.
I don't see Apple being big in the web space as much so Google really wanting to push PWAs vs Apple's stance is different. Although Apple seems to be fine with PWAs to a degree, so it would be the same for them too.
Apple really doesn't seem to care to support things outside their ecosystem (it doesn't benefit them like it does Google.) So you probably aren't going to see a multiplatform library from them.
Jetpack Compose is clearly a counter movement from Android team.
https://medium.com/flutter/announcing-flutter-2-2-at-google-...
https://www.press.bmwgroup.com/global/article/detail/T032861...
https://devclass.com/2020/10/29/sony-takes-a-flutter-on-embe...
https://stadia.dev/intl/es_es/blog/how-flutter-helped-us-mak... https://devblogs.microsoft.com/surface-duo/flutter-dual-scre...
I don't think the parents point is that all those are frameworks.
Rather that they have two many frameworks and platforms, with no coherent unified platform strategy (Apple's iOS, macOS, ipadOS, watchOS are all the same under the hood, and their UI frameworks are also unified or getting there fast).