The Rams couldn't run the ball well. You can see the Bengals were playing pass on that play. They assumed it was a play fake, i.e. more eye candy. McVay was counting on that surprise factor. It was probably his best call of the night but it wasn't the only one.
On the goal line on that last drive. Stafford's dive against a stacked box seemed to make no sense. I'm sure McVay told Stafford to run the play regardless of how the Bengals play it. It set up the next play in which McVay specifically aligned Kupp as the Z receiver. This gave him a two-way break and the previous play forced the Bengals to again play the run. The minute I saw Kupp in single coverage out there I knew the pass was to him and it was going to be a game winning TD.
Two calls showed me that McVay learned from his previous SB loss. It was his turn to outcoach the first-time SB coach across the field. It was clear Sean did not intend to lose again due to being outcoached. It's going to make McVay very hard to beat in the next Super Bowl regardless of who they play.