I have seen this. Some devs actually write a pure swift ui app and still manage to make that look like anything but a pure switt ui app. Since the mothership is Mozilla I won't put is past them.
Much more than UIKit, SwiftUI enables the web front-end mentality that mandates no stone be left without a custom style. It's one reason that I'm not particularly fond of it.
Thunderbird for Android is based on the 15+ year old K-9 Mail app while Thunderbird for iOS is a greenfield project.