He said it's more likely that a Calvinist believe God the author of evil than a non-Cal hold to open theism. But "more likely", even if true, doesn't justify attributing that idea to the whole. For example, I could say that a non-Calvinist is more likely to reject Penal Substitution Atonement (which is true). But I would be wrong to act as if non-Calvinists as a whole do so.
Even if 95% of Calvinists believed God authored evil (which I doubt is true), the fact that Calvinistic doctrine (the Canons of Dort) teaches otherwise negates such a claim.
@Revmitchell is right that some Calvinists demonize non-Calvinists. But he is wrong to go beyond Calvinistic doctrine (which specifically states that God is in no way the author of sin) in order to demonize Calvinism.