Exterior plywood vs sealed plywood
&quot;Exterior grade&quot; is confusing. The fact is that much of the plywood you encounter already is exterior grade, which (as you've stated) just refers to the adhesive used to bond the plies. It has nothing to do with wood preservation, for example. CDX is the rough grade I'd encounter in the U.S. for this purpose. The 'X' implies exterior adhesive.
Sealing plywood to achieve the same effect probably isn't a good approach. For one thing it'll change how your mortar bonds. For another, it isn't reliable as you'll be puncturing it with a hundred screws.