The only reason Dish knows when you're using a sling adapter on your 722 is because the receiver is designed to acknowledge it and your receiver reports back to Dish's servers that it has a sling adapter connected when it phone's home. The reason that's good is because then your dish.com account will know it's connected and let you access those features. Call center techs can also see it's connected for troubleshooting purposes.
In order for a receiver to detect a sling classic, it would have to be snooping on your network traffic and be looking for that. It may violate Dish's contracts with their content providers for you to use a sling classic on some technical level, so a Dish employee isn't going to encourage that. Also if that Dish employee can get you to buy another 922, then that'll help their 401k, ya know?