It was a little more than "slight." His TD percentage dropped by 1.2% while his INT% stayed almost identical. Completion percentage went from 70% to 65%. Rating dropped from 101 to 92. Considering the skill players around him, those are pretty solid drops.
I think their schedule had a lot more to do with his drop off in stats than a change in QB coach.
This season he played the Niners, the Browns, Ravens, Rams twice, the Cowboys, and the Bengals (when Burrow was still healthy too.)
Last season's schedule was much easier for the Seahawks.
It would make sense that a QB's numbers would look worse when facing better competition, particularly good defenses.