So is this replacing Tamriel Rebuilt or...
No it is not replacing Tamriel Rebuilt. Tamriel Rebuilt only concerns itself with mainland Morrowind. Every other province is being handled by other projects. Here is a list of the mods I know about that will be adding lore correct land: their titles and then what they are going to be adding when they are done.
Tamriel Rebuilt: The province of Morrowind which includes Telvannis on the eastern shores of Tamriel, the southern House Dres holdings that boarder Black Marsh, and the Morrowind side of the Valus Mountains (the natural boarder between Morrowind and Cyrodil), including the city Kogotel.
Skyrim: Home of the Nords: The province of Skyrim, including all of the "holds" there. This province is west-northwest of Morrowind.
ZackG's Argonia: the province of Black Marsh, which is to the south of Morrowind.
Province Cyrodil: This adds the province immediately to the east of Morrowind, Cyrodil. It is the province featured in the Elder Scrolls IV: Oblivion.