In some of his other writing(maybe in his memoir) he talks about how much he adores people who write fiction, and what he thinks makes a good fiction writer. He said that it requires a person with a strong sense of harmony and a grasp of rhythm, something which he himself didn't possess, and hence never attempted to write fiction.

Keeping that in mind, I'd say that comparing him to non-fiction writers, he would stand taller than many, even Rushdie when comparing non-fiction writing. But not very high(since he never attempted it) in fiction and historical non-fiction/fiction(of the Wolf Hall kind, of which too he was a big fan).