Q: language is fluid, so if enough people use "chat" as a pronoun wouldn't that mean it is a pronoun?
A: yes, but people don't use it as a pronoun so it isn't one.
Q: are you trying to police how people use words?
A: no, I'm describing the ways words are used. if people were actually using "chat" as a pronoun I wouldn't have a problem with someone describing it as a pronoun, because it would be a pronoun.
Q: well maybe I do use "chat" as a pronoun. maybe my pronouns are chat/chats. are you saying I can't do that?
A: no, obviously you can do whatever you want. however, if you did hypothetically use "chat" as a neopronoun, it would be third person, and this isn't even remotely similar to the use case being described as a "fourth person pronoun" by the post I'm talking about.
Q: is "we" a fourth person pronoun?
A: no, it's first person plural.
Q: is "chat" a second person pronoun?
A: that's a case, not a part of speech. any noun can be "a vocative" in the same way any noun can be an object.
Q: does "chat" mean "y'all"?
A: so close! "y'all" is a pronoun which, being a pronoun, can take the place of a noun in a certain context. "y'all" is second person plural, so if the context "chat" is being used in is that you're talking to a group of people (real or hypothetical) that you're calling "chat", it can be replaced with "y'all". however, this doesn't mean that "chat" and "y'all" are synonyms, that's just a pronoun doing what pronouns do.
Q: everyone who's calling it a fourth person pronoun is joking, so you shouldn't take this so seriously
A: if they're joking in a way that's literally indistinguishable from spreading Real linguistic misinformation the result is the same either way.