These are just 5xxxM under a new name, been around longer than you may think. Curiously I recall them being dx10.1 and not 11. Which would spec the 5xxx they chose were previously 4xxx in all but die size.
There was a minor riot at the time about AMD renaming a previous generation, not as big if they were another company of course. Makes perfect sense to me as they're perfectly good gpus and have been labeled accordingly to compare with the new ones to come which would go in the higher end laptops.