You ever heard of telling someone what they need to hear so that they'll man up?
Yeah, just like that.
Like I said you are looking too into this. And when Yoda says someone is ready, they are ready.
Luke only needed an X amount of skill to defeat the "enemy" of the Jedi, Vader. After Vader, Luke did not need to became the "jedi" that were defined in the PT. Eventually the EU would fill the void and Luke became more of a demi-god than Galen Marek was in video-game form.