It's not my argument: Jack Kirby, Steve Ditko, Alan Moore, among many many others have made this argument questioning whether Stan Lee can be said to have actually created these characters, if was not more an editor than a writer.
As Riesman points out that there's no contemporaneous hard evidence that Stan Lee created these characters. There was never any script and we basically have to rely on Stan Lee's word that he developed these ideas. His own interviews in the '60s has him discussing stuff in terms of editorial prompts and he himself said that Jack Kirby was practically a writer alongside him.
The question isn't that Stan Lee had nothing to do with the success of these comics, the question is can we credit Stan Lee for creating these characters or having any say in originating and generating these characters and stories. If we want to credit him for dialogue it's worth asking what people mean by that. Marvel Comics in the '60s were largely action and plot driven and certainly not as driven by dialogue as anything by Moore, Gaiman, Bendis, Gillen to name a few.