If he spoke up, it would have ruined to book. And the book was his creation.
Meh. Maybe it would've ruined it in 1960. But sometime between then and the 25 years that passed before Capote's death a lot changed. Maybe some of you -- who are able to see things through the perspective of a gay man -- can shine some light on the thought process there but I see it being worth finding out exactly how ruined my book might become. What's the worst that happens, he's not given credit for something he wasn't given credit for already?
Surely, if he wrote the story knowing what a masterpiece it was, his own narcissism wouldn't allow him not to preserve SOME sort of verifiable evidence of it being his creation.