The most important question for every cross platform framework is what happens to the UI?
Adobe products (both the Creative Suite, and their Flex Builder environment for Flash app) had their own design system that felt foreign on every platform it shipped on. If you wanted something that felt native, you had to reimplement e.g. Apple Aqua in Flash yourself.
Flutter goes out of its way to do that work for you, aiming for a "Cupertino" theme that looks-and-feels pixel-perfect on iOS.
React Native tries to delegate to platform primitives for complex widgets, so scroll views still feel like Apple's when on Apple's platform.
Just about every top-level comment here is talking about that in one way or another; yet the blog post doesn't mention it at all.
It's possible that Apple/Swift's mindshare among developers will lead to a significant number of apps shipping the Swift version for Android even if it means using Apple's UI, simply because they can't be bothered to make something bespoke for Android. Then again, Apple takes so much pride in its design language that it might not be willing to implement anything that feels good on a platform they don't own. If they were to ship an API-compatible widget toolkit, it might e.g. use intentionally bad spring physics to remind you you aren't on an iPhone.
I wonder how big the community part of this is. Is this an open source project of non-Apple people who are trying to break Apple's platform out of its walled garden? Is a lot of it funded by Apple? Ultimately, that's going to shape a lot of how this plays out.