Consider the following predicate:
The English language.
In knowing what the predicate means, you concomitantly get information that such a thing exists. Behold! An Ontological Argument! The same would be true of other predicates: vision (since you read it) reading (ditto) meaning and, as Descartes figured out, self or person or thought. Even if we grant that no predicate considered formally contains information about whether it exists concretely this does not rule out other features of a predicate that can provide information about concrete existence.
This is relevant to Anselm’s proof since it is not limited to a consideration of the predicate that than which, etc. or greatest conceivable being as taken formally but also includes a reference to the one who hears his argument and understands the terms and the referent. This makes it perfectly analogous to the Ontological Arguments given in the first paragraph where, in diverse ways, the mode of knowing the predicate provided information about its existence, even if, like Kant, we insist that an existential judgment is never required from any predicates considered formally.