I believe he could have said it. he was very much anti-war (with US) until forced to launch the attacks. He had been to the US and England. He did not fear our existing Army or Navy, but our industrial ability to REPLACE war materiel when needed.
Another quote attributed to him at the beginning of the war but not verified is "I can run wild for six months … after that, I have no expectation of success".
Interestingly, the battle of Midway, which most historians agree was the turning point in the Pacific war, was almost exactly 6 months to the day after Pearl.
Yamamoto is fascinating.