What do you mean by "equate", and why is that a blunder?

You mean to ask "what does CJ Date mean?" I'm just paraphrasing him. The blunder is to try to treat object classes as relational tables, as opposed to treating object classes as data types (domains, or "column types") in the RDBMS. Of course no SQL DBMS on the market supports proper user-defined domains, so there's no way you can do what he wants with any existing product (which is part of his point ;)

I'd recommend The Third Manifesto (make sure to get the 2nd edition, not the 1st) by CJ Date and Hugh Darwen if you're interested in more details.