The bolded part didn't happen because Mimasaka supposedly predicted everything Souma did, from the oxtail, the extra meat, the improv, and the garnish. This means Souma's attempt at hiding anything failed.
My proposal would be that Mimasaka thought it was garnish, but was in fact a completely new dish, and Souma would explain it as such. Even if Souma's final dish is essentially the same, in this scenario, Mimasaka made a mistake. That italicized part is critical in convincing the audience of the win.
And take note, I made this proposal way before we even began arguing about nonsense.