He wasn't, this is correct. And his "grunge" style wasn't really that innovative It was largely borrowed from The Melvins and The Pixies among others. Nirvana just popularized it.
But you cannot deny how much of an influence Nirvana/Cobain had on guitarists in the early to mid 90s. Just listen to the Stone Temple Pilots, Live, Bush, Candlebox, Silverchair Even bands like The Cranberries (e.g Zombie), and I would heavily argue The Smashing Pumpkins, Green Day to an extend(but they were mostly copies of The Clash). Also their Seattle contemporaries Pearl Jam and Soundgarden ventured into the "grunge" sound a bit in their later albums. (Vitalogy and Superunknown, respectively)