This is total conspiracy theory. They were best pals until her book hit the bestseller list and continued far longer than In Cold Blood. Then he never bothered with her again. It wasn’t like Capote to hid his light under a bushel - particularly if he was responsible for an American classic.
And people who are crying on tv about the “new” Atticus: he’s a fictional character, people!
I thought Mockingbird came out several years before IN COLD BLOOD. I know the Mockingbird movie came out first.