I think it was meant more as a message to Stalin.
It absolutely could have been intended for Stalin. If you think about the precarious situation sending fleet units into Tokyo Bay, you’d want it understood that those units have top cover. The IJN did have Kaiten (mini-subs) & motor-torpedo boats that could have been rigged for suicide runs. They’d been husbanding this type of equipment to repel the expected amphibious invasions. The USN would really need to discourage that thought by throwing “The Big Blue Blanket” over the area. If Stalin’s representatives were on the Missouri to witness the firepower, so much the better.