As others are saying, in gender critical feminism,(1) Sex is the name for the immutable fact of the body, while Gender is the limiting social constructs layered on top of it.
So 'gender abolitionist' is not the same as 'I don't see colour'. It's more like anti-racism.
Firstly, because it is challenging the limiting social constructs around the physical fact of sex, and secondly, because it's not ignoring the impact of sex or gender on our lives by pretending they don't exist, it's highlighting gender as something significant and damaging that should be got rid of
The "I don't see colour" line of thinking aligns more with the genderist belief that body sex is an irrelevant detail that has no significance or relevence to people's experiences and outcomes and so there is no basis for sex-specific rights or support.
(1) to be clear, this used to be just mainstream feminism - it is only since the idea took hold with some feminists that trans women are literal women, and with that the need to "respect" gender as superceding sex, that the label Gender Critical was coined