The ChatGPT app for Mac seems to be based on the same codebase as their iOS app (either using Catalyst or SwiftUI), so the lift to bring it to Mac was probably much less than creating a fully-native Windows app. Even today their "Windows" app is a wrapper around a web view.
Entirely AppKit? Do you have a source? I'm skeptical because it seems to behave like an iOS app ported to the Mac and has the exact same interface layout as the iOS app. Beside that, maintaining a separate AppKit implementation would be a lot of extra work that doesn't seem worth it (though of course, if anyone has money to burn on that, it's OpenAI).
I suppose it's more likely that the main chat interface and other highly custom views have separate implementations on macOS and iOS, while the Settings view is shared SwiftUI code (since it's just a lot of buttons and toggles).
