He's mad about terminology. The implication is that "waifu" should be reserved for Japanese media.
Despite the fact that "waifu" is a stereotypically Japanese pronunciation of wife, which is an English word.
Despite the fact that we're on fucking 4chan, and use terminology derived from unrelated topics all the time without complaining about the word being "appropriated."
These complaints pop up now and again. For some reason. I guess it's a weeb thing.