That's such BS. He changed nothing, except maybe what was trendy for a couple of years. METALLICA killed 80s hair metal, not Nirvana. Nirvana had nothing to do with metal. Grunge petered out in a short few years, and gave us bands like Creed and Nickelback.
And all that is just talking about children's pop music. It has nothing to do with REAL music, music for adults, which was completely unaffected by grunge or metal.
I have to Metallica was relatively unknown until 1990, when "One" hit MTV and "...and Justice for All" hit the store shelves. Sure, the serious music fans knew about them all the way back to "Kill 'em All" and "Ride the Lightning," but most folks in the mainstream hadn't ever heard of them.
Guns n' Roses, on the other hand, released Appetite for Destruction in 1987 and it broke through big in 1988....two full years before Metallica 'broke' into the mainstream.
Regardless though, I love both bands' music.