I think the 13 year old daughter, rolling her eyes and going, "Oh mummy...." hammered that home....
Saying that, while this article is complete bollocks, and I refuse to accept that she or her dd would actually have been fine had a 6'2 man with stubble (given that a trans person can be at any stage of their transition journey) wanted to start measuring her breasts, I do actually think this particular incident has been over egged.
Just asking if they needed help isn't in itself inherently sexual or intimidating. Nobody has suggested the staff member was actually fitting bras or supervising the changing rooms or anything that would involve any nudity or potential for intimidation.
I'd hope even the most strident GC person wouldn't suggest that trans people can't work as sales assistants etc. generally. In which case what difference is asking 'Do you need help,' on the sales floor to asking it (or a similar q like 'did you find everything you needed?') behind the counter or at the till?
Anyone, male or female, should be able to answer stock related questions like 'Do you have this in a 12?' Or 'where are your.....' doesn't matter if '.....' is bacon or bras. Acting as though it's somehow embarrassing to ask a man a benign question is in itself sexualising what should be a non-event. There wasn't any obligation to engage with the staff member. Saying 'No we're fine thanks,' isn't humiliating or awkward, doesn't matter if the person asking is male, female, trans, or identifies as a unicorn.
What would they have done if the person at the till when they came to buy the bras was male, if anything vaguely related to underwear or bodies is apparently so cringeworthily feminine only XX chromosomes should be allowed anywhere near the vicinity of the shopfloor? Same when the dd needs to buy tampons? Or get the pill from a male pharmacist? Or talk to a male doctor about her pregnancy?