Yes, D&D is the prize jewel, and they're particularly harpy-like about it. Modern, Past, and Future are not the same thing, and IP for them is not considered as important. Honestly, between say, Forgotten Realms and Shadow Chaser, which is the most valuable? Which is the most unique?
So, yes, they're more lenient toward d20 Modern products. While they don't want anybody else to benefit from the established D&D brands (Forgotten Realms, Planescape, etc., and now Eberron), they wouldn't object that much to other people supporting the Modern settings that are not really as successful as their D&D settings are.