_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)

They are pretty xenophobic, if you are Chinese or worse Korean you might as well be a gaijin.

Ha, or worse, Japanese-American.

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.