It got nothing to do with the sea devils Shanks talked about.
When
@Shudk made that devil fruit theory he specifically mentioned Dante's Satan, which has three heads and three pairs of bat-like wings.
Hito Hito no mi: model akuma is a more appropriate name.
Normally jolly rogers tell something about the captain (ie, Shanks' scar, Luffy's hat etc...) but Xebec's silhouette clearly didn't have horns, so it could only be an indicator of his ability.
Xebec also wanted to take down a group that called themselves gods, whom the D. on the other hand are their natural enemy.