I mean, I don’t think it was quite as prevalent as you’re making it out to be, yes there’s Ditko but I wouldn’t describe Kirby or Lee or Marston or even Finger and Kane as Objectivists even slightly. And even with Ditko, there’s strong arguments to be made that while he was at Marvel, he hadn’t yet become a full-on Objectivist.

Spider-Man’s credo of “with great power comes great responsibility” and his origin story of selfish self-promoter shown the error of his ways by the death of his uncle Ben; the original Doctor Strange is so uncompromisingly committed to the Hippocratic Oath that he won’t even allow harm to befall even Baron Mordo. (Even in his full-on Objectivist phase, Ditko tended to be more about moral absolutism – absolute good vs. absolute evil – than the pro-selfishness stuff.)
Where I think confusion tends to set in is that people associate the heroic individualism that is in superhero comics DNA from Superman onwards – the idea of a costumed do-gooder who steps outside the law or at least the institutions of society to set right what is wrong – with Objectivism. As you suggest, anti-altruism is at the heart of what sets objectivism apart from bog-standard individualist liberalism, and the heroic part of super-heroism is all about helping others full stop.