if you cared to look at your own link, you'd see that it is about patching files without the need to reboot. On a running system the kernel is entirely in the memory.
Also the statistics Microsoft is giving aren't impressive at all. From your link:
The following examples demonstrate possible savings from reboot reduction:
* Of the 22 updates that shipped for Windows Server 2003 RTM between April 2005 and August 2005, 15 of them required a reboot. Eight of these could have been hotpatched. This would have reduced the number of reboots by 53%.
* Of the 14 updates that shipped for Windows Server 2003 Service Pack 1 (SP1) prior to August 2005, ten of them required a reboot. Four of these could have been hotpatched. This would have reduced the number of reboots by 40%.
It seems to me, after some googling, that while Windows actually does have mechanisms to patch functions of the in-memory kernel and libraries, that what hotpatching means in the context of Windows is pretty much a normal upgrade without a reboot.