In the particular example he describes, both are placebos, because the performance is essentially random, and even in that case, it will appear that punishment

In the particular example he describes, both are placebos, because the performance is essentially random, and even in that case, it will appear that punishment is better than praise, because the administration of punishment/praise is linked to performance, which in this example, is purely random. His thesis is that praise, in real-world situations, has a positive effect, and punishment has a negative effect, but on extremely short timeframes random effects dominate, leading to the reasonable empirical belief that praise is ineffective compared to punishment.