The licensed SDK costs $12500. Start saving.
You will NEVER get the SDK by other means. Even if you did, didn't hold the license and released content based on that code, you will end up in court.
The developers of the SDK do not mess about. This much we know.
P.S The difference here is that Bethesda created their own SDK and can do with it as they please. UBI did not, they bought it.