For those who don't get it, Haruhi Suzumiya is the title character of one of the most influential and well known stories in the otaku community, but said story isn't mainstream by today's standards.
So if you claim to be a fan of anime/manga, but you haven't even heard of this character, then you're likely actually a fan of MAINSTREAM anime/manga which is just a subculture of the entire thing. Thus, any and all opinions you might have would be limited to that small part of the community and have less weight when talking about the culture as a whole.
The otaku community, at large, doesn't want all anime/manga to be like mainstream anime/manga.
This is the equivalent of claiming to be a superhero fan but your only exposure to them is MCU films, so you critique all superhero media as though they were MCU films while claiming to have a good grasp on the hobby as a whole.
This mentality is very annoying to anyone within any hobby and is a big reason why gatekeeping is becoming more and more accepted. Hobbyist don't want tourists deciding what needs to be changed about the thing they love.
