Dude's sitting there handing out sentences to the CDs. I don't anything less than a high ranked CD would be allowed to do that. The CDs that were targeted during the GV incident were probably the Figarland family.
So unless the CDs are just recruiting regular King's into their ranks, and giving them the title of Celestial Dragons, then I don't see how it is remotely feasible for him to be a Celestial Dragons, because they all abandoned their Kingdoms, and gave leadership to someone else over 800 years ago.
The only family that stayed behind and didn't go to Mariejois and become a Celestial Dragon, is the Nefetari Family. They are they only exceptions.
So unless more information comes to light how it's possible a King from the outside World was also a Celestial Dragon all along, the only assumption to be made, is that he isn't a Celestial Dragon.