The same thing could be ask, "wouldn't the Jews at hte time, Paul especially, have determined if Jesus were a 3rd person of a Trinity Godhead and said it somewhere?"
I agree, which is why I don't presume either one, either way.
Edited to add: Either of them could be right, or both could be wrong - which is why, imho, neither one is worth arguing over. Do we really think Jesus would want us arguing over it? Or would he want us to follow Him, and listen to his words?