I am a HV and I can say categorically that they CAN smile this early as I've seen them do it. Not just windy smiles but proper "let's copy Mum/Dad/big sister/brother" smiles......definitely NOT BU and don't let anyone tell you different.
You can definitely tell the difference so put it in your baby book! Who knows if the really early ones are 'wind' anyway - maybe they are just practising for the full grin. Why should it be 'wind'? My very pragmatic MIL said "It's just wind" and my very romantic FIL says "She is having a beautiful dream".