Step #10 – CURING: Let the shower pan set for 24 to 48 hours to allow the silicone to properly cure. Once cured, the wallboard and wall panels can be installed.

^^You might have missed this (from your link).

Solid surface shower wall panels are only 1/4" thick, I believe. And you need a surface to glue them to. So they must be backed up with some kind of sheathing.

I installed my solid surface walls in my shower over drywall. Unlike tile, solid surface is virtually waterproof. No additional greenboard or Redguard is necessary.

I trust the Corian. I don’t trust the plumbing. I wouldn’t put drywall in any wet wall plumbing situation. If the shower arm or valve leaks, you’ve got mush behind your Corian, and no way to repair it without a remove and reinstall. I’d recommend cement board as your backing. It will provide structural integrity even if you have a plumbing drip. An access panel to the shower plumbing isn’t a bad idea either if you can manage it.

Thanks for those answers. I did indeed miss the sentence in my link that said wallboard *and* wall panels. Oh, I get it now. By installing the pan all the way to the studs without backing, then putting cement board in on the studs and finally the solid surface panels ... if water ever gets between the solid surface and the cement board, it runs down to the pan anyway. Well, no, I still don't get it - if the solid surface seams are all watertight, water that ran down between the cement board and the wall panels would reach the pan ... and go around the pan to the subfloor instead of into the pan.