I think he meant the Japanese underestimating the U.S.'s ability to stick an army in their (Japan's) way.
No. I was referring to an apparent belief that the the Soviets could not stick a strong army in their way. And that is not to say that the Red Army was weak, but rather that it's a huge logistical effort to deploy a major army across Siberia and keep it supplied. And for what gain? The land wasn't of much value to the USSR, so would they fight for it? The Japanese may have made the same mistake they later made with the US and assume the answer to be no.