The author of the article says
The danger for trans women is that if wearing what are traditionally women’s clothes becomes the norm for men too, then trans women will no longer be able to rely on these props to help them display a female gender identity - and for many, that could be a serious problem.
They then ask 'Where does that leave [trans people]?'.
The answer is - Realising that genderism is built on sand. That in fact 'gender' isn't innate and certainly isn't within the control of an individual. Instead it's an experience co-created in the interaction between an individual's experience of themselves and the responses of others around them to their external presentation, as mediated through culturally widespread stereotypes around gender.
In other words, 'being' female gendered depends on others 'recognising' you as a woman. It's not just something you 'feel'. This is only possible in any way for someone with great hairy ham hands and five o'clock shadow by invoking stereotypical 'female' dress, ie putting a skirt and lipstick on. So, then, when someone else disrupts those clothing rules by putting a skirt and lipstick on but - shock horror - still seeming quite happy to think of themselves as male - then it reveals that 'being' female gendered in the sense that trans people depend on is ephemeral, imaginary and lacking any basis in fact. Worse yet, it reveals that the locus of this 'being' female gendered is not, in fact, the feels of the trans person claiming it but the field of culture, social stereotype and ideology - exactly where the feminists said it was. Oh, wait, unless 'being' a woman actually depends on, y'know, having female biology. Which you insisted had nothing to do with being female gendered, right?
So because you can't possibly allow behaviour that reveals 'being female gendered' to be pantomime, socially constructed and vulnerable to parody and disruption, it must be suppressed. I wouldn't be surprised to see trans activists campaigning to stop people like Jaden Smith and Eddie Izzard wearing what they like, because men-wearing-skirts-without-being-trans is 'transphobic'
Indeed, the NUS has already banned cross-dressing for 'shock value'. So, in other words, the trans lobby must campaign for tighter enforcement of stereotypical clothing for men and women, so that 'gender' can be foregrounded at the expense of sex or comfort, or choice, or sanity, or anything really.