It's been the norm when people don't know the sex of the person, or they're talking about a person in th abstract. However, if you know so much about grammar, you'd know that the rule used to be that one should refer to an abstract human as he/him. If you thought about it a bit longer, you'd understand that using they/them as singular pronouns for specific people is confusing because those words mostly commonly refer to more than one person, and there is no grammatical indicator to parse the difference.
Pointing to this colloquial use of they/them in the singular is just another "language game." It points to an innane truth and uses it to deconstruct existing categories that are otherwise logical. This is often accompanied by the motte and bailey tactic: changing the precise meaning of the term to be more or less radical depending on the stakes of the argument.
Eg. Nobody, I mean nobody, is saying that males are literally females; don't you know the difference between sex and gender...followed shortly by, how can transgender women have male advantage when they're women? Kafka traps!