I'm trying to find his post about this but I couldn't so I resorted to AI. Basically he has this theory that the D is half a moon, that they're half human and half moon people. It links to the Skypiea volume where Luffy appears to have wings and is winking at the viewer - and incidentally, apparently Luffy has never winked anywhere else in the manga. Anyway, he writes a column for the WSJ site. A theory column. He'd never had a theory rejected but this one was, and it stands to reason because he'd hit the nail on the head and they didn't want to post what were basically spoilers.