Firstly, Christopher is not dismissive of life-extension techniques that transhumanists espouse. If he could live longer right now due to a technique formerly classified a "espoused by transhumanists," he would choose to, as is evinced all over that essay. Rather, he is largely dismissive of life-extension techniques that don't work. The transhumanism part has nothing to do with it.
In that essay, cryonics is lumped in with the paragraph that discusses bad advice. So we can agree that Christopher considers cryonics as bad science, the same way he considers eating the granulated essence of the peach pit to prevent overwhelming growth of a malignant neoplasm bad science.
On the other hand, the rest of the essay discusses good science, that is, science which has been proven to work. He discusses its "near-miraculous" quality.
Perfusion of chemicals to preserve and prepare brain tissue is already textbook science, just not applied to whole brains. The case of Clive Wearing is one piece of an assortment of evidence that consciousness is a clever illusion (the philosopher of consciousness, Daniel Dennett, can say a lot here). Ultimately, cryonics is science that (by definition) has not been proved to work, but nonetheless serious science. Please don't deny that. Harvard neuroscientist Kenneth Hayworth explains this much better than I could, so I leave the floor to him:
http://brainpreservation.org/documents/killed_by_bad_philoso...
This issue is much too important to worry about what's distasteful.
I should have been clearer:
"Christopher Hitchens believes that most of the techniques that transhumanists think will lead to radically longer lifespans will not work."
I think that we are not in disagreement.
I think the supposition that he saw death from aging as unpreventable is supported by his lack of explicit advocacy for anti-aging research and by his public dismissal of cryonics. But that's probably because the current evidence is too fuzzy. We don't have the ability to bring back a cryonics patient, and we do know the damage involved in preserving them is non-negligible.
If he actually saw reversible human cryopreservation in action, with negligible damage, I doubt he would have abstained from taking a ten or twenty year jaunt to the future to see if they have a better treatment available for his cancer, or any other disease of aging.
Because when you think about it, that's a surprising thing about humans in general. Why do people who do not believe in immortality (as an ideal, I'm saying) still go in for cancer treatment?