You are confusing formats and concepts there. A 18mm is a 18mm in whatever format. The angle of view will change according to the format used. A 18mm angle of view on the 35mm format is a ultra-wide angle, but when employed in a smaller format as in the Pentax D-thingies, is equivalent to the angle of view that a 28mm would give you on 35mm.but as I said the '18' is really a '28' when used on full format 35mm cameras.
Your sentence would be correct if it reads this: "but as I said the '18' is really a '28' when used on APS format cameras."