Upstreamable means adding it to the official Linux source being distributed on linux.org.
It just means you have to download drivers separately until Nvidia give the kernel guys a pull request for the drivers to be included in the kernel sources by default.
I think the more important thing than the separate download is that when someone works on the kernel, they usually make sure that the don't break other things that are in the official kernel. But there's no obligation not to break things outside the official kernel (apart from the interface to userland).
It just means you have to download drivers separately until Nvidia give the kernel guys a pull request for the drivers to be included in the kernel sources by default.