You are right about Sosa Sensei technically not having rank because of founding the IAA.
His last rank conferred was Rokudan by Roderick Kobayashi Sensei when Sosa Sensei was still a top instructor for Seidokan.
You are also right about his Godans.
But, on the IAA site he still refers to himself as a Rokudan. And in his office he has his Rokudan Certificate. I may be mistaken, but knowing Sosa Sensei it is probably out of the amazingly high level of respect he holds for his instructors (Koichi Tohei, Isao Takahashi, and Roderick Kobayashi)
P.S. Joe, did I meet you at Sensei's surprise birthday party?