Well Vasher is able to suppress his Returned aura, perhaps Elantrians just haven't figured out that particular trick, or maybe Hoid is just far enough away.
On the other hand, I'd be kinda surprised if he actually was Elantrian, but perhaps there is a way to acquire some of their powers in the same way that you can use Lerasium or be an Awakener/5th Heightening without being Returned.