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As to the nature of the error when 'add_subparsers' is given an 'action' parameter:
'add_subparsers' does several things to 'kwargs' before it passes them to the relevant Action class.
 def add_subparsers(self, **kwargs):
 # adds 'parser_class'
 # removes 'title', 'description' (used in an argument group)
 # add 'prog'
 parsers_class = self._pop_action_class(kwargs, 'parsers')
 action = parsers_class(option_strings=[], **kwargs)
What I wrote earlier about using the registry is partly wrong. The Action class is determined by either the 'action' parameter or the registry entry.
 In [17]: p._pop_action_class({}, 'parsers')
 Out[17]: argparse._SubParsersAction
 In [18]: p._pop_action_class({'action':'test'}, 'parsers')
 Out[18]: 'test'
So the 'action' parameter works - if you specify a compatible Action class.
 sp=p.add_subparsers(dest='cmd',action=argparse._SubParsersAction)
Such a class must have the same __init__ signature, otherwise you'll get errors such the OP's.
It might be worth rewriting the documentation line so this is clearer. Otherwise I recommend closing this issue.
 action = parsers_class(option_strings=[], **kwargs)