_Gaijin_ is commonly used for non-Asian foreigner. Take a look at how citizens of non-Japanese heritage are treated if you want to understand what the parent comment is referring to.
For example, Korean or Chinese heritage Japanese, or Ainu (indigenous Japanese)
I don't think the parent comment was alluding to only foreigners: https://en.wikipedia.org/wiki/Karoshi is one of the unfortunate social phenomena that's notable in Japan from the culture and the pressure of conformity.