I think it might vary from creator to creator
Like the people who worked in comics from the 40s to the 60s were science fiction fans, and possibly also read pulps and comic strips, and probably read a variety of stuff outside of that.
The generation Moore refers to might be like the generation that had Roy Thomas, Gerry Conway, Len Wein, Marv Wolfman, Steve Englehart, Steve Gerber, Chris Claremont, etc but I'm pretty sure many of them read more outside of comics. Like I'm pretty sure Thomas and Claremont definitely read a lot of books even if you don't think they're Moore-level writers.
I know Byrne can be comparable to Johns in terms of being fannish, but I can actually believe Byrne reads things outside of comics. He was a big science fiction reader when he was younger, and I can believe that from seeing his work.
But there are some creators don't really have much reference point other than comics themselves, or worse, extremely shallow stuff from pop culture to draw from. It's like the difference between Lucas' Star Wars (OT and Prequels) and the Star Wars Sequel Trilogy. Even though the Prequels have serious problems, it and the OT show Lucas clearly had a lot of varying interests. Like in ANH you can see he enjoys Kurosawa, westerns, Flash Gordon, WWII dogfights, and other stuff. In Force Awakens you only get the impression that its primary source of influence is only the Original Trilogy.