He deleted his old messages and edited which he can't delete. His rank should be recalculated in a proper time.
IIRC you will not receive a demotion if you were previously a newbie a hero member or legendary if you have lower activity because of deleted posts.
The person in the OP appears to have deleted several hundred of his posts (maybe more) and more often then not this is an indication that he is trying to cover up some kind of scam, or evidence of him scamming.
This also happened to TheGambler although he was a hero and not legendary.
Edit:
IIRC only Newbie, Hero, and Legendary members don't get demoted normally. Maybe I'll fix it at some point, but it's not really a big deal.