Smiling will cause you to sing with dreadful tone - you'll sound overly bright and harsh.
Lifting the cheeks won't change your placement either - these are myths that don't hold any anatomical water. When vocal teachers tell you to smile when singing, what they're aiming for is false vocal cord and external muscle retraction to avoid compression around the vibrating folds.
Try this - imagine you have to laugh without making any noise and without releasing air (you can imagine that you have a friend the other side of a plate glass window and you want them to see that you're laughing, but you can't let the people in the room know) - you'll feel your throat open. Now place your fingers either side of your adam's apple and slide down to just below it (this is harder to describe for women ) - if you do the same exercise, you'll feel the throat muscles move aside.
That's what the smiling is for, at least anatomically speaking.
Oh, and - leave your eyebrows where they are - there's nothing worse than the smacked clown look on a singer.

EDIT - forgot to actually answer the question - if the smile / laugh technique is being used to remove constriction in the throat (which implies it has been correctly studied as a separate technique until singers can feel the retraction happening) then it will help people avoid singing flat because they will require less air to make their notes, less air means less cord resistance and less likelihood of bowed cords - therefore less likelihood of a note going slightly flat. I would say that he's right that smiling, if well done, can help - but that one has to know what one is doing and what one is supposed to feel when one does it - it's perfectly possible, for example, to do an insincere smile, which won't move anything in the throat.