Testing with Manual Dependency Injection according to Google

There is an article from Google about Manual Dependency Injection. In it, they use an Application class that holds an AppContainer, which itself holds dependencies that other classes might need:

class AppContainer {
    private val retrofit = Retrofit.Builder()...
    val userRepository = UserRepository(localDataSource, remoteDataSource)
}

An Activity might then do the following to get a dependency, eg. a repository:

val appContainer = (application as MyApplication).appContainer
val loginViewModel = LoginViewModel(appContainer.userRepository)

They state the following:

Dependency injection ... enables you to swap out implementations for testing.

Can this be done with that approach (or any of the others presented on the page mentioned above)?

I tried to come up with a canonical way and didn't find one that wouldn't involve reflection or mocking or anything of the sorts.