A great part of its import and significance stems from the large number of phenomenon that quite fairly fall under its name.
Some of Brands most obvious failings in the past may well fall under a range of labels that go beyond what you see the core of sexism to be. Thats your problem, and failure of others to adhere to these restrictions does not mean other people are 'watering down' the term sexism in a manner that harms the cause, no matter how loud or how long you bleat on about it.
Whatever labels people find most appropriate, clearly Brand has done things in the past that he shows at least some signs of being ashamed about these days. I'm not sure if he will manage to convince all of his critics that he's really got a clue on some of these fronts now, perhaps because the way he speaks and writes tends to add a sort of fickle, glossy objectivisation to bloody everything. So even when he's trying to declare that he now stands against the sexual objectification of women, the declaration itself seems to end up encapsulated in a shiny bauble.