It's not true that they would know whether it was a placebo. Placebos in psoriasis trials have a very high seeming response rate -- either because psoriasis waxes and wanes on its own, or because the placebo effect is mysterious but it is indeed something that DOES cause improvement in many patients.

To say that someone in a clinical trial improves so therefore they could not have been on a placebo is an UTTERLY FALSE statement.