I think the advice was sound. For an unfit rider on a short commute, a MTB with slick tyres and the suspension locked out will be fine. As he gets fitter and decides to commit to this on a regular basis, it may well make sense to change to a hybrid,CX or road bike and get the benefit of the lighter, more agile bike. But as things stand he'd be spending the money for not much increase in speed or comfort.
Incidentally, what does this mean?
... the pannier touring guys can't lane split up to the lights with me.