Near glacial maxima, archaic humans could have walked to Japan, so they surely did. Elephants made it.
Most of the time, even in the ice ages, Japan was fairly isolated. Maybe the northern path through Sakhalin was open (although glaciated) , but most of the time water separated Korea and Japan.
There must have been versions of homo erectus and Denisovans in Japan. They haven’t been found, but I think people haven’t looked much.. It used to be assumed nobody lived in Japan before the Jōmon, so people didn’t dig before the Jōmon stratum (14,000 BC).
But there must have been archaics there, back in the Paleolithic.
Seek and ye shall find.