Well, one rather famous Tesla hacker did exactly that, but he said the wiring challenges were extremely annoying and he wouldn't do it again.
Presumably the post refers instead to Tesla's rather unique practice of including the full hardware set on every car but only giving you the supporting software if/when you pay for it.
I'm of really mixed minds about that. It simplifies the build process and gives base cars more future upgradability, but if a lot of folks aren't paying for the upgrade it seems like Tesla is wasting a bunch of money in parts that never get used or paid for. (This is made more complex because some of the AP hardware is used for included safety features on all cars, including those without the other features.)
