True. MS is basically trying to gloss over the fact that so much of the base code for 8.1 is demanding so many patches that they had to rewrite it, but rather than calling it something like Windows 8.4 or something they decided to call it Windows 8.1 "Update". But the 8.1 Update is not accessible at their main site which is causing some problems.
It is not a COMPLETE rewrite of the base code, but it is substantial enough that you are looking at a whole new install rather than a security patch suite a la XP-SP3.