Sounds like a load of bull shit to me. Beginners are beginners. Skis or snowboards, they both suck at their respective sports. The reason it seems like snowboarders are a bigger problem is because snowboarding has recently made up a dominant percentage of new participants at ski areas. Regardless, most of these beginners stay away from steep bump runs.
Moguls are out of vogue. I think modern grooming equipment is the biggest factor in the reduced number of mogul runs (thank god). The ungroomed (ungroomable??) bump runs remain in tact (upper nash), but runs like triple 60 are now groomed nightly.
Name a few ungroomed bump run that have suffered from the advent of snowboarding.