I don't think this is considered a "leak". I mean, it could totally be "true" but I believe lists like these on nvidia's servers are... how do I say... "predictions" in their own rights. I mean, I'm a web developer and there are plenty of sites around here that prepare articles and content for things that may never come.
Why do news sites post articles 3 seconds after a celebrity is confirmed deceased? Because they write the articles potentially years in advance just in case. After all, they gotta be the first to publish.
At my job, I build web pages and products on a dev server that may or may not ever "go live". There's proof in the database that they exist, but it doesn't mean they'll truly be "real".
If development studios are anything like run of the mil websites, then they have "templates" ready for all potential games and projects that might never reach the public.
I feel all this really confirms is that Square has started working on KH4 internally. It could be as simple as they copied and pasted the KH3 engine and renamed it to KH4.
Also, potentially, the KHX, UX, etc stuff could have been for internal development use as they created UX/Dark Road/Chi the browser game. It doesn't immediately mean there will be a remake or PC release, but it's possible.
So anyway, the KH4 thing is probably real. It's not really surprising or a leak since we knew it was coming and Nomura stated he wanted this year to be fancy. I think the UX thing might not be what people think it is though, but hey. I'd be surprised to see otherwise.