Sounds like a short cut. The mud doesn't have squat for adhesive characteristics. There are fillers on the market made just for this. They are harder to work with than drywall mud which is why he wants to skip it.
It has to setup and then get sanded, it sets up alot harder than mud which you can flake with your finger nails.
Lots of companies make the stuff.
You never get the fruit if you don't go out on the limb!