Fair enough--he hasn't proven he can consistently play like he did today. But most "talking heads," here and elsewhere, said he couldn't do half of what he has already done. How many playoff wins did Cutler deliver in Denver? Orton?
Lots of people expected him to play poorly and that's exactly what he did. You are attributing the team's success to him. Mark Sanchez has more career playoff wins than Matt Schaub. This is not because he's a better quarterback. It's because of coaching, supporting talent, and luck. This is not really a difficult concept to grasp. Trent Dilfer won more Super Bowls than Dan Marino. This is not because he's a better quarterback. It's because he had the benefit of playing against a relatively weak NFC contender on a team with an incredible defense. Again, correlation vs. causation. Tebow's presence on an 8-8 playoff team does not mean he caused an 8-8 playoff team.
It's not really possible to quantify a quarterback's role in his team's win-loss record. We don't know where the Broncos would be without Tebow. Maybe they're winning because of him. Maybe they're winning in spite of him. The fact that his passing numbers rank near the bottom of the league suggests to me that it's probably the latter though. I think this is a case like the Sanchez Jets or the Dilfer Ravens where the circumstances have been so favorable that virtually any QB could yield decent results with this opportunity. That doesn't mean the QB gets all the credit.
Put Matt Moore or Andy Dalton on a strong team in a cake division with a cake schedule and you'd probably get something like 8-8 as well. Doesn't mean they're great players or that they're proven anything. The 49ers were 13-3 this season. That's an elite record. Does that make Alex Smith an elite QB? No, of course not. He's only one piece of the puzzle and it's highly likely that the team would've produced similar results with any number of other players under center.
Overall, Tebow played poorly this year. That his team has still managed to win some games and luck their way into the postseason doesn't make him a proven commodity or a respectable NFL QB.