I think the Japanese in the series are the descendants of Japanese management and engineers, pre-apocalypse who ran the factories for the likes of Toyota, Honda, Mitsubishi etc.
As most of these companies seemed to have built their factories on the East Coast its logical they would end up in the rust belt.
Its been a while since I read the series but I seem to recall that they had fled Japan.
I did many years ago write a story based on a European expedition that has set out to discover what happened to North America
I cannot recall all the details but it involved a super sub capable of operating small helicopters and the explorers used a fusion powered armoured buggy with a rapid fire rail gun
The short story I wrote had one such buggy with a 2 man recon team coming across a 'largered' Amtrack
1 of the men while collecting water comes across a similar party dong the same thing.
Lots of shooting first and not asking any questions on both sides.
It was much cooler in my head as I was writing it.