They get angry because the “I identify as an attack helicopter” meme so perfectly skewers their argument of “you are what you say you are, and no one can disagree”. Comedy and mockery have the ability to punch through nonsense far more powerfully than screaming abuse or even rational argument. Someone screaming slurs in your face is easy to dismiss as a “bigot” and it also serves to impart the victim status and the inherent power of victimhood on you. Someone laughing at you and mocking your position through jokes is far more difficult to deal with, and laughter is infectious. Good jokes, like Ricky Gervais’ jokes about “old fashioned women, the ones with the wombs versus new women” hit hard because they point out the inherent ridiculousness of the trans position. Reductio ad absurdum is a powerful logical argument, and their reaction to it demonstrates that.