To determine whether a company is undervalued you must have some prediction of future business prospects.
Basically you could bid on some esoteric future or you can bid on the bad present.
This is the difference between someone who had a good life and education and suddenly ended up in a car crash and lost all of their family vs someone who decided to be a meth addict and ended up killing their family.
In both cases you’re bidding on their future and in both cases they are in the same position, but it doesn’t make them equivalent.
The performance of the Nikkei is significantly more complicated than just a low interest rate. For example. the lack of productivity and population growth in Japan is another set of pieces that also impacts the rate of return.