Some parsing of the question and tendentious semantics here I think - a license is used to legally fly an airplane - I think that is the best definition of "valid" - OK play ducks and drakes with the medical requirements if you like but get ramp checked in the US with a piggyback and no medical from your originating ICAO state and you'll be getting a big fine, possibly some jail time, a life-time ban from the US and, if the check is following a accident, US level law suits coupled with invalid insurance.
The rules are very simple and are set out in the regulations with commendable clarity - no reason to be imputing ambiguity to them where there is none.