I would be surprised if Apple isn't working on SwiftUI for web themselves. They have web versions of all of their major apps already – Photos, Numbers, Keynote, etc. I'm sure they'd prefer to not have a completely separate implementation for those. SwiftUI currently supports all the platforms those apps... except the web.
Also it makes a ton of sense to have an Apple-supported way of writing web backends in Swift... after all, they have a lot of those too.
I don't know if they'd support Windows and Linux because they could just support web instead and that would be good enough. Maybe Android is worth it though? The main reason is I'd imagine they might want to unify the codebase of Apple Music and Apple TV apps, but I'm sure they'd worry about bringing too many good apps to their competitor's OSes. That's probably too scary so they'll probably prefer to just build two apps like they already do.
Also, this reminds me, is there any way to listen to Apple Music or watch Apple TV on Windows, Linux, or Chrome OS? I don't think so. I'd have to imagine that's in the works.