This kind of statement give credits to those who say that's actually the same game despite what your real thought might be.
Because yes everything that's built is built on its foundation. And even besides the map, a lot of thing are straight up from BoTW, from items to monsters, to music, to how the content is delivered and structured...even the gameplay, the UI, the animations, weather effects, organic interactions, things you find in the world (mazes, dragons etc).
So yeah, on those terms, i guess it's fair to say BoTW was a demo, but it's not a compliment to ToTK, just as much as it's not a compliment to BoTW. It's the first time i have this feeling with a main Zelda game (it's not the case at all for Majora's Mask and yet that was developed very quickly) and it seems everybody is fine with it, even trying to make up the idea that the game is so much better than BoTW (i mean yeah i sure hope that when you take basically everything from a game and add 6 years you can do better). The main concerns i see are related to the game not being like a traditional 3D Zelda game, which i don't really care since i love BoTW.
That's still a very good game but i don't really share the excitement. The additions are pretty good but still additions. The powers have huge potential but honestly neither the shrines nor the map from BoTW really exploit ultrahand at its full potential, far from it. The dungeons i did are also pretty weak.