Even Roger, Rayleigh, and Gaban defended the Celestial Dragons from Rox. Saint Shanks having some kind of loyalty/agreeance with them would not be surprising at all.
I 100% believe Saint Shanks’s sword has the Gryphon fruit just like Shamrock has the Cerberus sword. And the Celestial Dragons wouldn’t give Saint Shanks a powerful sword like that for no purpose. Saint Shanks definitely has loyalties to them.
One Piece fans aren’t ready for it, but Luffy and Saint Shanks will end up on opposite sides of the EOS conflict.