While that may be true, you are leaving out the rest of the story about what actually happened (instead of focusing on what should have happened). Based on the player package the Seahawks had on the field, BB thought had a high probability that SEA was in a passing set. Based on tenancies and film review, he snuffed out what the play call was likely to be.
The Pats had practiced that same exactly down, formation, and situation for two weeks leading up to the Super Bowl. Malcolm Butler said he got verbally undressed in practice by not doing what Bill wanted in practice while working on that exact play call in practice. Low and behold, SB on the line, Butler's eyes lit up when the exact formation that he had been abused on in practice presented itself. SEA confirmed the play call of a slant when the RB they had motioned out of the backfield. Butler adjusted where he was standing (the Seahawks had stacked receivers on the right side) to get away from the oncoming block and to get closer to the path of the football before the snap and jumped the route.
BB also confused SEA by letting the clock run. The Seahawks were SURE the Pats would call timeout to try to give Brady one last gasp to try to pull the game out with seconds remaining. That altered what the Seahawks could do, as they had to try a pass play or they ran the risk of the game clock expiring if they didn't get a clock stoppage.
And statistically, Lynch that season was not a sure thing at the goal line. While people tend to think that the Seahawks would have scored if they inserted Lynch and their goal line power package, IIRC Lynch was a 50/50% proposition or worse in goal line situations that year. BB later said had the Seahawks put in their power running package, he would have had to call time out to get in his run stuffing goal line defenders.
Like always, we can always guess what could have happened, but we only know what actually happened. I've heard BB and Malcolm Butler discuss this play in detail over the years, so while many others may just call it dumb luck, I call it master level coaching and preparation. You have to get in position to win, and that's exactly what NE did.