It's not a dick move because the singular "they" covers every conceivable pronoun that could ever possibly exist. Saying "they" is not the same as calling a man "she" or calling a woman "he" or any combination of genders and pronouns. The singular "they" is as accurate for one gender as it is for any other possible genders and covers all of them exactly the same.
That's the entire reason neutral words like "they" exist, to cover every possible case to avoid mis-gendering. If we decide that the singular "they" is inappropriate, we might as well do away with pronouns altogether and refer to people by name every time we mention someone.
English already has a non-specific pronoun that's been around for hundreds of years. It has come from the French, so it's been around at least since the Norman Invasion of 1066. So roughly 1050 years.
At least in modern English, "one" is specifically an indefinite pronoun, and is therefore inappropriate for use when referring to a specific individual.
And before anyone claims otherwise, "they" was still in use as a definite, gender-neutral pronoun long before its NB usage began to become mainstream.
"They" might be a definite pronoun, but it is almost exclusively used as a plural. It's also a pretty bad drop in replacement; "They is currently running" sounds weird as all hell for instance. That there has been an explicit and forced attempt to shift it towards that doesn't make it sound any better.
Furthermore, "He" has been used as a generic pronoun long predating the singular they. While it comes with some ambiguity as well, it flows a lot better and does not screw around with singular/plural ambiguity.
"They" has been used continuously for the singular since 1300. It has literally never died out, despite protests from grammatical prescriptivists beginning around 1795. Even Shakespeare used singular they, but this would have been considered unremarkable by his audiences.
The only thing that seems relative new here is usage to refer to a specific known individual, of known gender. Historically singular they was used definite but unknown individuals, or at least individuals of unknown gender.
In any case, "They is" is wrong for singular, just like "you is".
"You" always takes "are", for both singular and plural "you". Indeed singular "you" differs from plural "you" only in the reflexive: yourself/yourselves.
Singular "They" is exactly the same. It uses "are" and differs only in the reflexive: themself/themselves. The only difference here is that the plural reflexive can be used for the singular, and indeed that has been standard for several hundred years, but feels extremely awkward with respect to a known specific individual.
Fun fact: "themself" actually predates "themselves", and was originally used for both singular and plural they, although around 1450 "themselves" became standard for plural, with "themself" remaining standard for singular. But eventually "themself" almost completely died out.
That's the entire reason neutral words like "they" exist, to cover every possible case to avoid mis-gendering. If we decide that the singular "they" is inappropriate, we might as well do away with pronouns altogether and refer to people by name every time we mention someone.