I wonder if the apps are native or if they're using some cross-platform framework. I would assume Xamarin, but I could also see them using React Native because their desktop app is Electron and React.
There is some grousing[1] that a cross platform technology, including Microsoft's own Xamarin, wasn't used and that fully platform native development is a waste. Was it mentioned somewhere specifically (maybe during GH the keynote that I didn't watch) that SwiftUI was used? That would be the only way to square the tweeter's statement that only 20% of mobile users would be able to use it, he must mean only those on iOS 13, but not totally sure. Hightlighting Dark Mode and a good iPad version would play into SwiftUI's narrative but the framework still seems a bit new for an app of this caliber.
React native is an inferior product to IONIC when you want to have your own UX style which github probably want.
CSS is unmatched that's as simple as that.
Moreover, they would be able to reuse far more code.
[1] https://twitter.com/ThomasBurkhartB/status/11946837811956736...