I'm not entirely sure this is indicative of that, I mean what were they supposed to do? SwiftUI is clearly the right choice for iOS, but if they use it on Mac they would have to develop a separate implementation for older Macs, and anyway they hit problems where SwiftUI just flat out isn't mature enough on Mac anyway.
The only other alternative would have been to rewrite the native client again in ObjC, knowing that they would have to rewrite it again in the near future anyway. All of these options suck really badly. I think it's just unfortunate timing given the current state of development of SwiftUI. do you really see a better pragmatic way forward?
This way hopefully in future when older Macs cease being an issue, and SwiftUI on Mac matures, hopefully they will port the iOS code base to Mac. At least the fact they went with SwitfUI on iOS instead of a cross platform framework is a good sign.