It's not clear to me whether scientism believes that the mind is a process that cannot take place on any substrate other than a brain, or whether he's shares my and (I think) Mitchell Porter's more cautious point of view that our consciousness can in principle exist somewhere other than a brain, but we don't yet know enough about neuroscience to be confident about what properties such a system must have.
I, for one, would be sceptical of there being no substrate possible at all except the brain, because it's a strong unsupported assertion on the same order as the (perhaps straw-man) patternist assertion that binary computers are an adequate substrate (or the stronger-still assertion that any computational substrate is adequate).
If I have understood scientism's comments, they believe neither of the possibilities you list in your first paragraph.
I think they believe that whether or not a mind can take place on a non-brain substrate, our consciousness(es) cannot exist somewhere other than a brain, because they are currently instantiated in brains, and cannot be transferred (whether to another brain, or anything else).
This does not preclude some other mind coming to exist on a non-brain substrate.
In June 2012, Robin Hanson wrote a post promoting plastination as a superior to cryopreservation as an approach to preserving people for later uploading. His post included a paragraph which said:
This left me with the impression that the chances of the average cryopreserved person today of being later revived aren't great, even when you conditionalize on no existential catastrophe. More recently, I did a systematic read-through of the sequences for the first time (about a month 1/2 ago), and Eliezer's post You Only Live Twice convinced me to finally sign up for cryonics for three reasons:
I don't find that terribly encouraging. So now I'm back to being pessimistic about current cryopreservation techniques (though I'm still signing up for cryonics because the cost is low enough even given my current estimate of my chances). But I'd very much be curious to know if anyone knows what, say, Nick Bostrom or Anders Sandberg think about the issue. Anyone?
Edit: I'm aware of estimates given by LessWrong folks in the census of the chances of revival, but I don't know how much of that is people taking things like existential risk into account. There are lots of different ways you could arrive at a ~10% chance of revival overall:
is one way. But:
is a very similar conclusion from very different premises. Gwern has more on this sort of reasoning in Plastination versus cryonics, but I don't know who most of the people he links to are so I'm not sure whether to trust them. He does link to a breakdown of probabilities by Robin, but I don't fully understand the way Robin is breaking the issue down.