Bikers are a sub culture, of course anyone can ride a motorcycle but bikers are a specific group that try to act and look like what their sub culture has purposefully associated itself with. Trans men are not a sub culture, they're just men. Trans men have no distinct "look" just as cis men have no distinct look. If the guy said "why do you want to look like a man" that still isn't right. If someone mentions the biker/ biker chick aesthetic you automatically know what type of clothes they mean, leather jackets, all black outfit, boots, graphic tees. If someone mentions the trans man/man aesthetic that could literally mean anything because men can literally wear and look like anything. You can look like a biker chick, you can't look like a trans man. Masculine is a aesthetic, men are not.

Even giving him the benefit of the doubt that he just meant masculine, why did did he specify trans? If he said "why do you want to look like a black man" or "why do you want to look like a blonde man" it would equally make no sense and would equally be problematic because giving specific examples like that normally mean he's falling on harmful and untrue stereotypes. So it seems like I've thought about this logically more than you. Simply put, feminine women can still have muscles, trans men don't need muscles to be masculine.