J'onn's comment on choosing to look like that also makes no sense, he had to look like Hank Henshaw originally so he could keep working at the DEO to reform it from the inside. He couldn't really change his look before being found out because then he'd lose that position.
After he was found out he could have easily changed his took whenever he wanted, but before that, it really wasn't a choice.
Hence the reason his comment was nonsense; he will never know what it means to truly live as a black man, as his appearance is no more than a mask he has the luxury of removing whenever he chooses. Obviously, black people have a birthright / heritage / generational experience an alien will never share or comprehend, so again, his statement was BS, all for the show to use whatever was laying around to fit into this episode, no matter how inapplicable to the central subject.