I’m pretty sure that’s a bogus quote, created out of thin air and repeated a million times on the net.FWIW Yamamoto was educated at Harvard and was, oddly enough, pro American for the most part.
Here's two more of his quotes:
"In the first six to twelve months of a war with the United States and Great Britain I will run wild and win victory upon victory. But then, if the war continues after that, I have no expectation of success."
"I fear all we have done is awaken a sleeping giant and fill him with a terrible resolve." (on Pearl Harbor).
I still have seen no verified attribution for that quote. A letter, a book, etc.