This has nothing to do with biology, it's 100% English grammar. Biology books are not the ones explaining what's a "pronoun". In fact, many languages don't even have pronouns, and yet their biology books can perfectly be translated.
English grammar is constantly evolving. In fact, it would make it a lot more realistic if there were more strange expressions and mannerisms, even outside of pronouns. Battlestar Galactica did add some funny nonsensical expressions like "frak", "rook", "frimp", "pogees" and popular phrases like "so say we all" that added a layer of depth to their futuristic culture, making it a lot more believable.
I don't think this mod really helps with "realism". It would in fact be unrealistic to expect no linguistic changes at all in 300 years. The only reason why I think some people are mad is because they think it's pandering to a political agenda so they don't even want to see an option be given at all.... despite it actually being a totally optional and even totally believable aspect of a future hypothetical culture. Whether you agree with the direction that this hypothetical futuristic culture developed towards or not.