This isn't about 'men wearing bras' or men wanting bras advertised for them. It isn't about trans women wanting bras to be advertised for women and trans women (trans women would be included in the 'for women' already)
This is about young trans men, who are probably feeling incredibly self-conscious about their changing body, who may be experiencing dysphoria around developing breasts.
They will be going to shops and having to deal with the same problems faced by the younger sister of Yellowberry's founder, the problems that led to the company being created.
So they go to Yellowberry to find a nice bra that won't emphasise their breasts, that will actually provide comfortable support without drawing attention to them or giving them unwanted cleavage. However, they then have to deal with constantly being referred to as girls/young women/daughters, which will then make their discomfort 10000x worse. Their options then are to go to specific websites that provide undergarments for trans men (binders, packers etc...) or use completely inappropriate things like elasticated bandages that can cause serious harm to them.
What does it hurt for a brand to just change their wording to include anyone who needs a bra to support developing breasts. Girls and women are included in everyone, but so are people with breasts who ID as men or as nonbinary. All it does is make it easier for the people that might otherwise find it distressing to exist in their current physical body.
Maybe, changing wording in this way would make it so less people feel the need to medically transition in order to cope.