Wow, I hadn't thought of the Kirby claim to have created Spider-man and how Ditko would have reacted to that. But by Stan Lee's account in ORIGINS OF MARVEL COMICS, he was the genius who came up with everything, and just farmed it out to Ditko to draw Lee's preconceived vision. I'm sure Ditko has a very different view of that. Ditko, at least later on, was very active in plotting and developing characters, and likely from the beginning, beyond the lack of credit acknowledged by Stan Lee.

Kirby said he came up with a similar character years before, and I've often seen an unpublished cover for it from 1954, titled NIGHT FIGHTER, that Kirby alleges was re-packaged into what became Spider-man.
This much is true, Kirby did the cover for Spider-man's first appearance in AMAZING FANTASY 15. So who's to say that Kirby might have had additional conceptual contribution to Spider-man early on. But I suspect this is either defective memory on Kirby's part, or his early ideas were vastly changed by Lee and Ditko.

You bring up a great point that Kirby was very liberal/left, whereas Ditko was on the polar opposite end of the spectrum, as an ardent conservative and Ayn Rand/Objectivism follower.
But I have many friends who are liberals despite my own conservatism, and you can share many other interests while disagreeing politically, or even just not discussing politics. In the far more civil (relatively) 1990's, I had a very liberal friend who was a news reporter, and we had lunch several times a week together, and I actually consider him my best friend from that period.

I think that Kirby and Ditko were introverted and not particularly social would have been a more likely barrier to their being friends. But it's still possible they were friends. They were the two giants of their time at Marvel, from roughly 1958-1966, and in Kirby's case 1958-1970. It was only when Ditko died that I realized how many characters he's created, at both Marvel and DC, as well as at Charleton and other companies. I think Ditko is the only other artist who has created characters on a scale as prolific as Jack Kirby.