I don't understand why people keep calling this a Windows XP update. It's not. It uses the "Microsoft Update" automated update system to update a Microsoft product, namely IE. It's not an update to XP. It's an update to IE.
Because if it were classified as a Microsoft update for IE, as you say, covering all installations of IE 6 through 11, then those running Win98/ME/2K/pre-SP3 XP, with IE6, would also receive that update, just like XP SP3 users with IE6...and they are not
. So this is an XP SP3 update.