The docs correctly say that usesService should be called for tasks that use the service.
But this does not work and is not tested.
If you add the call to the compat test, you will see that you get complaints about the provider type.
Even if you don't do it like in the documentation but use the original provider from the extension, you get the same build error.
You have to give the original provider you got from the register call to the usesService method.
But in the extension you only provide an arbitrary provider (actually, a property) that is set using the provider from the register call.
But you have to provide the original provider via the extension and change the readme to advise to use the original provider, not the property of the task class.