Article use with job title (a sentence from a book)
I'm upgrading a comment to an answer as suggested by the OP. I apologise for the lack of supporting evidence which, with the reference to KateBuntin's comment, was the reson for my not posting it as an answer initially.
They could have said "Ronald Hutton is a professor of history (at the University of Bristol). That would say that he was one among the international community of professors of history. But that would have required both the articles I have inserted. As @KateBunting has said, for a unique post capital letters are most appropriate.