> this argument defaults to "None"

Am leaning to the fact that this works as required. The documentation does not just say defaults to none. It says:

With the 'store_const' and 'append_const' actions, the const keyword argument must be given. For other actions, it defaults to None.

In the scenario, you have given, you are using 'store_const' so the const keyword argument must be given.

This should be closed If you agree with what I have just stated.