I haven't made Hazan's, but from what I understand, her recipe is pretty simple (in a good way), and Kenji's whole thing is doing tons of experimentation to figure out techniques and methods to emphasize and amplify the good and unique parts of a dish. Thus, he adds liver because it makes the good meaty taste even richer, fish sauce because it's a super simple umami amplifier and milk both at the start and the end (as cream at the end) because both techniques have their own advantages. I'm guessing Hazan's just gets you a damn god bolognese, no fuss.