I think people would actually be surprised to see his post-up numbers. Pleasantly surprised.
Not only does he have a higher FG% (in post-up situations) than Duncan, he was at one point in the top 10 (8) of all NBA team leaders in post-up FG%. He has some glaring weaknesses when it comes to the post, but he also does some things as well (or better) as anyone in the League. You're right about the sample size, it's not very large. But his post-up attempts make up 33% of his offense so I think there's still an adequate number to start drawing conclusions. For example, he's only converted on 30.4% of his hook shot attempts from post-up situations. And he shoots the hook 60.5% of the time while posting up. One thing he does extremely well in the post is getting to the rim. Whether it's his up-n-under, back-shoulder spin or quick dribble drive, he uses his footwork and gets to the rim very well when given enough time and an adequate clear out. Even though he hasn't been great with his hook shot, he's still managed to shoot 42.1% in the post. Compare that to Tim Duncan - 38.1% in the post and All-Star Marc Gasol - 45% in the post.
A post-game is something you develop over time as familiarity (with opponents and positioning) increases. He needs reps to hone his skills. Marc has over 200 post-up attempts; Tiago has 79.
Those stats are accurate as of right now (excluding the Mavs game). I have quite a few more Tiago stats I pulled from Synergy a few weeks ago that I need to update in Excel. Whenever I get a chance I'll update them and post them in a thread. Some of the data is pretty interesting.