SwiftUI previews work simply because Apple provided the support in UIKit and SwiftUI so that we can quickly view the result after we made changes to the code (it's almost similar to using Simulator). Same as UITest, this approach can still be previewed and tested that way.

But for unit test, since the logics are inside SwiftUI view struct, it will be technically impossible to test using unit test. The only possible scenario for unit test in MVVM is to move the logics outside the view and put it in an object, the standard MVVM way (Like in the second code snippet in this article).