He was at his confirmation hearings, or at least he portrayed himself as an originalist. If Kennedy and the dems hadn't disgracefully treated Bork as they did, the American public would at least have the democratic process solve what the Constitution didn't specifically address. Yes, Souter voted on the
right to an abortion, school prayer, gay rights, etc. That's the social liberal trying to make the case that he's for less government intervention. However, he also voted with the liberal wing against school vouchers, the death penalty, and to weaken national security law. He's voted with the liberal wing on federalism, term limits, affirmative action, racial gerrymandering, and church-state relations. Conservatives who describe themselves as social liberals believe that less is more. They believe in less government intervention to force socially conservative results, but they don't believe in a ham fisted federal government forcing the liberalization of social policies. Souter was a solid liberal. Your article mentions his respect for precendent, but that was only when precedent yielded liberal policy results. The only thing that differs him from the Warren Court is his lack of heavy handedness.
Also, progmatism isn't a good thing when you are a judge. To paraphrase Roberts, your job is to call balls and strikes, not offer an assist to your philosophical brethren.