I can't imagine it to be true...he was so outspoken, yet he never made the claim himself.
They did find a letter he wrote to his aunt before publication:
He was petty, jealous of the attention she received.Dated July 1959, a year before the novel was published, he told his aunt that Lee had ‘showed me as much of the book as she’d written and I liked it very much. She has real talent.'
I don't buy it one bit.