What Eliezer is talking about (a superintelligence paperclip maximiser) does not have a pleasure-pain axis.

Why does that matter for the argument?

As long as Clippy is in fact optimizing paperclips, what does it matter what/if he feels while he does it?

Pearce seems to be making a claim that Clippy can't predict creatures with pain/pleasure if he doesn't feel them himself.

Maybe Clippy needs pleasure/pain too be able to predict creatures with pleasure/pain. I doubt it, but fine, grant the point. He can still be a paper clip maximizer regardless.