What did Ma Plank note about Daddy that Xuma could not believe in the novel, Mine Boy?

From the text:

He had never seen Daddy sober. He had never seen Daddy stand upright without swaying from one side to the other. And yet Ma Plank had said he was a man such as she had never seen. And it was true, for in her voice had been the ring of truth and in her eyes the light of truth.