Tenacious D has sold millions of records too. And tenacious D has plenty of riffs that could put megadeth and Metallica to shame. KG can shred, son and Jack Black is a sic vocalist. There is nothing illegitimate about that.
Have you ever watched any of the "Guitarings" videos that KG does with John Konesky (D's electric guitarist)? They're entertaining and informative. Check 'em out on Youtube if you get the chance.