I see this stated as gospel, but I have never seen anywhere in the old books where this is true. Yes, AD&D (and Greyhawk) has a lot of inspiration from sword and sorcery, but it pulls plenty from other fantasy as well. Races, rangers and monsters from Tolkien. The monk from kung fu movies. All manner of monsters from Greek and Norse myth. Vampires and werewolves from gothic horror. I'll admit my knowledge of Greyhawk is limited to 2e and later, but I've never seen this "S&S theme" expressed. Not in the way Eberron pulp or Ravenloft horror is expressed. The first time the notion of Greyhawk being the S&S was expressed overtly was in 5e.
Which is why I find it to be more a retcon than a design paradigm. If there is some grand statement of intent that Greyhawk was supposed to be S&S flavored D&D, it a) failed and b) is well obscured. What Greyhawk feels to me is a kitchen sink setting inspired by the stuff Gary was familiar with, just like every other DM creating a home setting. My first setting was inspired by Star Wars, X-Men and Final Fantasy stuff, but I would not call it a "Sci-fi/comic book/video game" themed setting.
If you have some definitive proof Gary wanted Greyhawk to be the S&S D&D setting, I'd love to see it.
