Okay, but how bad is that, relatively speaking? Let's imagine income in Japan doubled over night for everyone. You'd still have that same rate of poverty. We need to look at the outcome. As the article says, you don't find drug addicts and petty criminals roaming the streets in Japan.
This number also disregards price differences. If you are a young worker, you likely live in the city, paying exorbitant Japanese urban rents, requiring a commensurate salary. If you are old, you likely live outside the urban center on a modest pension, with far lower living expenses. Japan of course has a very large elderly population.