I don't think the author ruled out a field-theoretic description (whether as a quantum field or as a classical field). But using the language of fields and talk

I don't think the author ruled out a field-theoretic description (whether as a quantum field or as a classical field). But using the language of fields and talking e.g. about the precise coupling between gravity and other quantum fields doesn't add anything to the discussion here. The question is whether the gravitational field behaves in a quantum mechanical fashion (and thus allows for superpositions) or not. To ponder this, you don't need fields. It's a completely separate issue.