I don't quite follow. You think that a BJJ practitioner who includes striking, clinching, and takedowns in his training will have more of a problem in a real fight than one who only practices groundfighting?
Speaking as an amateur Sumo practitioner, I think Sumo can help develop certain skills and attributes which are useful in a fight. But that's not what it's for. Sumo practitioners are training to compete in the dohyo, not worrying about "hey, this wide stance leaves me open for a groin kick" or "I need to work more on my knife disarms."