I always found this argument funny because how would you use pronouns for someone whose gender you do not know? They. It's they. E.g. you are given the sentence: Jordan went to the store to buy apples. And you want to ask a followup question regarding how many, you reply: How many apples did they buy?

And that's not how English was taught to me or 99℅ of the population (including English as a second or third language) 20+ years ago. Singular they was only used for situations where the gender (read as superficially visible sex) was factually unknown. You see a forgotten umbrella and never saw who forgot it: "Somebody forgot their umbrella." As soon as you only got a glimpse on the person forgetting it you would make a guess about he/she.

You're contradicting yourself here. You're saying you were taught to use singular they when gender is unknown, yet claim that "How may apples did they buy" is wrong based on how you were taught English.

Does it matter whether gender is unknown or just unresolved? Not really, singular they is clearly intended to be a gender neutral pronoun and works in any situation where gender is ambiguous. It's not wrong for people to adopt it as a pronoun to refer to themselves any more than it is for a trans man to adopt "he/his" or a trans woman to adopt "she/hers."

At best your refusal to use it makes you sound like one of those people who gets offended at the word "literally" gaining a colloquial meaning that differs from its original definition. At worst, it presents as transphobia to claim "language purity" as the reason to be so adamantly against something that the trans community has largely adopted.