A dogwhistle is something that’s been used so much by a certain group (usually of hatred) that it’s gained the suggestive connotation of a bunch of arguments or a certain belief. It’s like saying black people like eating watermelons. Wikipedia describes it as “the use of coded or suggestive language in political messaging to garner support from a particular group without provoking opposition”. That’s the status the “attack helicopter” identification has gained, as it’s frequently used by bigots to mock the concept of identifying as a (different) gender.