The difference is that the new areas in Emerald and Platinum were minimalistic. They were the Battle Frontiers, the Distortion World and an extra couple of small caves in Hoenn. There were very few actual new areas. They followed the same narrative with very few differences.
This is set mostly in a part of Unova that isn't seen in the previous games, with some overlap where in the areas are radically different as two years have past since the story of Black & White.
The story is COMPLETELY different, not marginally different as per other third versions, the characters are different, gyms are different, gym leaders are different, available Pokémon are radically different. It's a new game, just set in the same region as before.
Yes, Kanto in GSC was post-game content, but this is on a much MUCH larger scale. Besides, we don't know if Unova is properly frozen over or if they're just hiding stuff from us, which they have done before.
Trust me, it's not Grey, or an enhanced or re-jigged version of Black & White. They are new.
Think of it like how XD was to Colosseum.