Because fixed, shared, numbered generations are a false re-writing of history by Wikipedia. Nobody used these numbers at the time, or at all until Wikipedia.Not saying I disagree, but this isn't a valid argument since by this logic the Master System would be a 4th gen system considering it's a successor to the SG-1000 which itself is a 3rd gen system.
The Mark III was absolutely a new generation for Sega. A huge leap in graphical ability from a Creativision/Colecovision clone to beyond the Famicom.