Strikes me as weird too but Williams Sonoma is wrong. I had a call from a Corian cutting board supplier recently. I told him I wouldn't allow them in the store for the reasons mentioned above. He said he had a study that showed Corian wouldn't dull edges any more than wood and wanted to send it to me. I told him unfortunately his study was wrong. I've studied it plenty myself.
I think you can buy a study that will say anything. I assume Williams Sonoma uses one of these studies to make their claim.
Nevertheless, Corian is a bad material from which to make cutting boards because it will dull an edge significantly faster than wood. How much faster? About 12 times faster. I say about because I don't have any scientific equipment for measuring wear, edges and things like that. I base my own tests on cutting into the cutting board enough times to require me to steel the edge. Not scientific but certainly close enough for government work and close enough for me to say categorically that Corian is way, way harder on knife edges than wood is. Glass and granite, of course, are even worse. I can't imagine how you would rig a test to show Corian was no more abrasive to edges than wood. I guess I should contact the guy and ask him to send the test information.
The rule of thumb is that wood is best. Bamboo and polypropelene are the only other acceptable alternatives. I usually recommend people get a good NSF rated thick wood cutting board and then buy a poly board to use for things like cutting raw poultry or garlic. Just set the poly board on the wood board and then throw it in the dishwasher or sink to wash when you've finised with it. The wood board will handle most of the cutting chores. Cutting boards, by the way, aren't very expensive. Anybody can afford a good one. Take care.