There's no "Knock Out" dwarf --yet-- but I have run across people who use the term Knock Out as if it were a generic term identifying any red rose, or any shrub rose. It's really really possible the people at the store had signs mixed up.
How far north are you? I know for sure they don't get as big further north. Maybe 3' tall, where down in Louisiana I saw 8' monsters. They get to about 4' where I am in Kansas. How big are yours?