Their source claims that Windows XP has ~45 million lines of code. But that was 14 years ago. The last time Windows was even in the same order of magnitude as 50 million LOC was in the Windows Vista timeframe.
EDIT: And, remember: that's for _one_ product, not multiple products. So an all-flavors build of Windows churns through a _lot_ of data to get something working.
(Parenthetical: the Windows build system is correspondingly complex, too. I'll save the story for another day, but to give you an idea of how intense it is, in a typical _day_, the amount of data that gets sent over the network in the Windows build system is a single-digit _multiple_ of the entire Netflix movie catalog. Hats off to those engineers, Windows is really hard work.)