Correct. There's a bracket you need to add to the 73-76 fenders to mount the earlier grille. Also I believe if you use the earlier headlight buckets you have to drill a new mounting hole or two. It's a fairly minor deal to use the '73-76 fenders for a '72.
This is the bracket needed
@Kern Dog has done this swap, he may have more insight. But it's pretty easy to use a 73-76 Duster fender for a '72.
Not at all true. You add the bracket for the upper grille support and everything else works just fine. The Duster/Valiant fenders have very few changes from '67-76. Marker lights, a bracket or tab here and there. Adding the bracket would be easier than changing the marker lights.