Except they do. Google Pay / Wallet is built in Flutter, and so was Stadia. It's a bit nonsensical to think that they'd rewrite every existing app they have in Flutter, even if it is their own toolkit. Google doesn't believe Flutter is the be all, end all in app development. But it is certainly the best cross platform app toolkit out there. But guess what, Gmail and their other apps don't use a cross platform framework, so the statements "Swift UI makes it easier to brand your iOS app" and "Flutter is a great cross platform app framework" aren't mutually exclusive, hell Flutter can even use SwiftUI views as part of it's Widget hierarchy via PlatformViews.

This post comes across as yet another person that's never actually investigated a tool casting aspersions on it because of what they hear online.