Speculation aka headcanon
Sengoku is only ever in the entirety of One Piece history said to be a CoC user in a
single Vivre Card, which have had incorrect information many times, which is why I said maybe, what don't you understand about that?
Imu can gift and take immortality and he's connected to them on a mental level, it's not an outlandish claim at all, especially when Saturn is a pathetic bum and yet was hyped up to have strong haki, it would make sense if the haki was just handed to him by Imu and Saturn himself was inept at using it.
Garling is Shanks' dad, that's all I need to know to say he's got CoC.