These kinds of claims tend to always be overblown, as I understand it. It's kinda like when someone sues a company for five-hundred million dollars. Realistically, they'll only get a fraction of that, but hey, ya never know. In any case, since Kirby had claimed creation of Spidey, it had to be included. Personally, I think he thought he created Spider-Man, but Greg Theakston did a pretty in-depth investigation on the subject and it was more than likely Jack was getting some pre-Marvel characters that shared similarities with Spidey (Night Fighter, The Fly, Silver Spider) conflated with his drawing the first appearance cover.
It's not without reason to see why he might think that.