They say it in the show, too. Tyrion says it to him, that Jon was lucky to be trained by Winterfell's master of arms Ser Rodrick (In the first season). Jon is supposed to be an excellent swordsmen, with the skills any knight would posses. He didn't get chump training, he got the same kind of training people like Jaime or The Hound would have gotten. In addition, unlike most tournament knights, which follow rules and mostly fight in "games"? Jon has actual combat experience now, far more than most people. There is no reason to believe most of the "professional" knights are better than him. In fact, from what we've seen; many of them are significantly worse than they boast.