Well actually they're both right to a degree
There are both soft and hard tippets
The major difference between fishing line mono and tippet is the tippet is more consistant in both diameter and strenght.
A leader built with hard mono will turn over better and hard tippet will not collapse under the weight of a heavy fly.
But the last thing you want when fishing a dry fly, or a nymph for that matter, is a hard tippet that lays out straight.
The s-curves in a good dead drift are the result of a proper slack-line cast and the use of a soft tippet