Well, this does raise an interesting question about what makes one a true "Jedi" - is it the completion of some rigid set curriculum of physical, mental, and spiritual training, with every i dotted and every t crossed and every checkmark duly checked off? Or is it the attainment of something that is not so easily quantified and measured? A particular quality of mind, heart, character, that can only be proven to exist at any given moment by the words and actions of the one in possession of it?
Luke had already faced what surely was the most rigorous and gut-wrenching test of his "training period": he'd learned the worst part of the truth about his family heritage, realized that he was the son and heir of the (second) most evil person in the galaxy (and remember, Luke hadn't met the Emperor yet, so in his mind at the time Vader *was* the ultimate evil), discovered that pretty much everyone he'd ever trusted had lied to him about it, and been tempted to turn his anger and resentment over that into a reason to transfer his allegiance from them to his own flesh and blood and go to the dark side. Which, you know, was good enough for dear old dad and those other people
did lie to you, son.
He faced that test, and he passed it. I think that's the yardstick, if any, that Yoda was using.
Does he have more to learn? Yes, because there is
always more to learn, even the "master," the
sensei, is just someone who is further along the path than you are. The path itself does not end and that is as it should be because some training can only be done "on the job" and some wisdom can only be learned - really learned and internalized - through personal experiences yet to be encountered.
In addition I think his entire attitude and behavior in the film that followed, in particular his final refusal to allow the Emperor to manipulate him towards the dark side by setting him up to fight and destroy his own father... that's plenty good enough for me.
