The XBone supposedly sold out at launch.
http://imgur.com/gallery/X3m4uW0
They haven't been "sold out worldwide".....ever. Microsoft is just flat out lying in order to feign high demand, so that people who don't do their research will buy one the moment it becomes "available" because "I have to get it now, or else I won't get it for a very long time", ultimately spurring sales.
Smart marketing move, but it doesn't mean they aren't lying.