I don't want to diminish the talent of the historian in vain but past performance is not always an indicator of being good at predictions, he might just have been lucky in a completely random guess 9 times out of 10. Given the amount of historians making such predictions it is not unlikely that such an historian exists and it would be fairly easy to mistake their success for talent.
I don't know where I found this but I found a scheme somewhere to scam some investors:
Find a large list of potential victims. Tell one part of it that you predict that market will do A, and the other part that the market will do B. Repeat the process several times, selecting only the investors to whom you've always told the correct prediction. Eventually you will have a handfull of people who have "solid proof" that you are a visionnary and you can scam them.
Again, I absolutely do not mean to say that this particular historian is bad. This story just reminded me of these ideas and I wanted to share.