That seems to be the case as tensei mentioned they have god's knights. Even akainu refrained from getting in their business as the knights had authorithy to judge even the celestials themselves so they surely should be powerful. The gorosei mentioned marines are merely their public face to justify their authority with a facade of justice. It can be similar to the royal guard in bleach that were completely seperate from the squads of soul society but if mihawk is one of them then he did a damn good job in hiding it as there are no panels indicating any kind of relationship between him and the wg other than the shichibukai status. I mean he has not made even a comment on the WG or the celestials and he is too young to be present in big time events surrounding the wg such as the mess in god valley
But the main reason is that there would be no reason for Zoro to face Mihawk except this
I think God’s Knights are the most elite celestial dragons and Imu is one of them.
Imu in my opinion didn’t exist 2 years ago, but rather he awakened from amongst the God’s knights sometime after Marineford to parallel Luffy’s awakening as Joy Boy