Probably because after Windows 7 they started a kernel rewrite , known as MinWin, to extricate the Win32 Userland tendrils that had crept into the Kernel since the NT days.
The article doesn't seem to indicate that MinWin started after Windows 7. If there has been a fundamental kernel change effort after Win7 (I'm not aware of one), maybe it has a different name?
You're right, for some reason my brain said Vista came after Windows 7. Given that 7 came after Vista my theory makes no sense.