How about the Thiel part ?
From Stereophile review of the classic Thiel model:
Let's just say that Fang's experience of speakers is extremely limited, but his experience of headphones is not. Since he doesn't sell speakers, that's not a problem.
Also, many people's association of "American" as an adjective meaning "stupid and unsophisticated" has some small basis in reality, but it has absolutely no basis in the world of audio, particularly when you are talking professional and high-end equipment like JBL and Thiel.
It doesn't matter if the classic Thiel model is extremely neutral and by chance Fang said HE400s are tailored after Thiel FR (which he didn't, he just wanted to say HE400s would be tuned after his idea of how American brand speakers sounded in general), all that matters is he has a predisposed idea of what tonality he wanted to tune these headphones into, and did it. Whether that predisposed idea is somewhat true, very true, or completely false is irrelevant.
Another reason might be that since Hifiman already has a very neutral can (HE500), they had pressure to make a lower-end planar model distinguish itself, through colouration.