As far as I can tell, they had very little time to test Vista out for themselves. During my experience with Windows, it's always fast and works properly right after a fresh installation, the problems come when I start installing new software or hardwares.
These sites have also come up on the front page while searching for mojave experiment:
http://blogs.zdnet.com/hardware/?p=2311
http://www.crn.com/software/209800292
According to the first article, it seems they didn't even get to touch it.