From talking to Ruby, the issue here is really about the fact that the officers do this to people coming out of Casa Ruby on a regular basis. D.C. police are pretty infamous for their treatment of trans* people in general and transwomen, in particular. Problems have included profiling transwomen as sex workers, neglecting to investigate murders of transwomen, and even shooting at them off duty.
This particular incident involved, among other things, being stopped on a freezing cold night for about two hours (why in the world did it take so long?) under orders to leave the car engine turned off (so there was no heat) as well as slurs and harassment. The misgendering of the arrestee (with whom I am also acquainted) was only the tip of the iceberg. But the misgendering does, itself, violate the general order in place on dealing with trans* people.
So in short, the issue here really is not about the idea that the arrestee was doing everything right and got picked on by the cop. It's about the police targeting a certain population (Casa Ruby clients) and mistreating them in the course of the incident. It's about bringing attention to a big, big, big ongoing problem with the D.C. police. There's a reason that report indicated problems in the ways the police deal with LGBT persons. This is just exhibit #18,451.