I think the answer is, both pattern and fabric give a quilt a modern look. I think of modern fabrics as having bright colors and a clean look, and modern patterns as using larger pieces of fabric, or going outside of the traditional format of repeated blocks. I agree with the idea, you know it when you see it.
I have used old-fashioned fabrics with modern (big piece) patterns and gotten some odd-looking results. Using modern, large print fabrics in a traditional small-piece pattern can be difficult, because you don't know what color will end up in your smaller piece.