The concept of "tomboy" in this comeback dosen't refer to the literal meaning of tomboy where the girls dress up in a boyish fashion, but instead is a criticism towards the differences in how boy groups and girl groups are treated.
I just rewatched the Tomboy MV with the lyrics on and I think I understand the whole message of not changing yourself for your lover (?) and it's kind of insinuating that they're not going to become these perfect doll-like women for him. Just like them embracing their tomboy (?) side because the guy doesn't like it???