@MainActor and the guarantee that the properties are only ever accessed from the main actor

In https://developer.apple.com/videos/play/wwdc2021-10019/?time=815 was said: "By adding the new @MainActor annotation to Photos, the compiler will guarantee that the properties (...) are only ever accessed from the main actor."

What does it exactly mean? Let's consider the following example:

@MainActor
class ViewModel: ObservableObject {
  @Published var text = ""
}
struct ContentView: View {
  @StateObject private var viewModel = ViewModel()
  var body: some View {
    VStack {
      Text(viewModel.text)
        .padding()
      Button("Refresh") {
        Task.detached {
          await updateText()
        }
      }
    }
  }
  func updateText() async {
    viewModel.text = "Hello, world"
  }
}

Compiler doesn't complain. After button tap viewModel.text is changed from background actor and there is runtime warning: "Publishing changes from background threads is not allowed; make sure to publish values from the main thread (via operators like receive(on:)) on model updates."

So once again: How exactly @MainActor annotation and the compiler will guarantee that the properties are only ever accessed from the main actor?