That's not true, outgoing_introduction_request is a defaultdict, hence this would still work.

If you change it to:
self._statistics.dict_inc(self._statistics.outgoing_introduction_request, destination.sock_addr)

The dict_inc internally does a:
self._statistics.outgoing_introduction_request[destination.sock_addr] += 1

Which is the same as you wrote on line 2421.