That's why I am saying, the Kamusari Saint Shanks used isn't the same as the one Roger used in the Oden flashback.
Kaido has multiple thunder baguas, Zoro has multiple sanzen sekai, Luffy has multiple pistol attacks. There's no reason to think that all the kamusari are the same and that the one Saint Shanks used to 1 shot Kid is the same one that Roger used against Oden.
Most likely, Saint Shanks used a stronger version of kamusari. This would spare Kid from total humiliation, not completely shit on kaido and BM, and give Saint Shanks an impressive feat in the process.