Steyn was referring to Michael Mann of hockey stick fame ... but yeah, hard to tell them apart.
Here's some of what Steyn's book says about James Hansen:
James Hansen was the most influential climate-change promoter pre-Mann. In June 1988 his dramatic testimony to the US Senate was reported by the following days New York Times under the headline Global Warming Has Begun. Certainly, the global-warming movement had begun. Hansen pushed the boundaries between scientist and propagandist, but, unlike Mann, he did not push the science itself into outright propaganda.