Dragon did not save Luffy from Buggy. The whole point of the scene was in establishing it’s Luffy’s “fate” to become Pirate King

Within One Piece, there are two incidents of miraculous natural disasters saving the Pirate King: the first one is in Loguetown, where Luffy is about to get executed by Buggy on Roger’s execution stand, before a bolt of lightning hits him and Buggy, miraculously saving his life.

The second is with Roger against Shiki, where, after being cornered by Shiki’s fleet, a storm suddenly wiped out his entire fleet, while Roger and his crew came out unscathed.

The point of these incidents is not that there was some secret devil fruit user who saved both of their asses, it was that there’s an invisible unknown factor behind the Pirate King’s ascension, whether it be fate or extreme luck. And it favors the D clan, and it’s needed to make someone the Pirate King.