It does work, you just need around 200 gb of “ram” for a 8k map, and with ram i don’t mean you actually need to have it as real ram, a big enough pagefile also works (google how to increase your windows pagefile size), it took me around 90 minutes, where my computer is totally unresponsive for the first 30 minutes, cause it is filling up the ram/pagefile, in the next 30 minutes the progress bar start to move slowly and you can use your computer a bit, this is where it takes the info it stored in the ram/pagefile and use it, then you also have to save the project to disk and that is another 30 minutes… Don’t know why they made that way, it’s very stupid to not take it in chunks and try to store it all in memory at once, but if you have the harddrive space (don’t have to be your default drive or a fast one) and the patience, it can be done… Just increase your pagefile (and if use a pagefile on another drive than the default one, set the one on the default one to a fixed size also, cause it uses that one first and if your system disk is small, it is going to fill it completely, and that will also make your system less responsive), go do something else while it works and only build nanite when your are done with your landscape 
Ohh and also, it does not clear up your memory and pagefile properly after it is done, so you also have to restart the computer after to get it fully back