I did check with Dave Behrens before I posted it, and he was surprised that they did eat snails, but when I sent him the link from Sea Slug Forum
(Robilliard (1971 observed it tending to specialize on either bryozoans or small snails in two different habitats, depending on which prey item was most abundant.), he did not call it BS.
Also Hans Bertsch included it in his new Invertebrates of Northwest Mexico, which is now at the printer to be published shortly.
I agree, it did not sound plausible when I first read it...