I don't see where he calls Steyn a liar, per se. I just read it last weekend. He just says that Steyn doesn't carry the argument all the way to any exact conclusion.
All true, inasmuch as Stein stops short of writing a full prescription for his diagnosis. Which he does, mostly. He only gives a glimpse there at the end of what sorts of things the prescription may need to do. But yes, it stops there.
I think it was intentional. I think the book was intended to get people to more honestly think about the diagnosis first, before getting lost in the decidedlly un-PC talk like "Steyn declares war on all of Islam"-- which is exactly what all the fuss would have been about had he done so. The strength of the diagnosis would have gotten lost in the hysteria over the conclusions.
That the conclusions are obvious serves the point without having to go around on the book tour getting distracted by them. You got it, and I got it, and this reviewer got it. Steyn hits on it here and there. I was struck by a quote that really hung with me... something like: "There may indeed be moderate Muslims, but there is no moderate Islam". Slam. Dunk. Undeniable.
It also lets Mark write a second follow-up book on the topic and go on another book tour, so... there's that.