I read that article the other day. It is an opinion piece, not the official view of SA - though I'm not impressed that they have anything to do with it at all.
My main thought was "So what if you can prove sex is a spectrum?" I mean we do argue that sex is fundamentally a binary, in a biological and functional sense, and that the existence of intersex doesn't change that (as some intersex people also say). But of course anyone who's wedded to this argument won't listen.
However, even is sex was a spectrum of some kind, that doesn't mean a man can just decide to be a woman. ASD is a spectrum, but that doesn't mean that if you're severely autistic you can just identify yourself to the other end of the spectrum - or vice versa. Hair colour is a spectrum, but if I have dark hair, however much I bleach it that won't make me blonde.
Race is also a spectrum, far far far more so than sex is. There are many different ethnicities and genetic groupings, and countless possible mixtures of those. And yet trans activists do not (or none I've ever encountered) think that you should be able to identify as black if you're white, and make out that you're black by adopting a bunch of stereotyped "black" culture and aspects of appearance, and then get to scream "bigot" at black people if they dare to question that you're really black.
So IMO the spectrum argument is specious anyway, so we can ignore it.