That's not really a great way to prove your point. You can't work backwards from successful cases without taking all of the data into consideration. Not every big that Manning coached at Kansas, Tulsa, and Wake made the NBA. Some have, however, and their developmental trajectories suggest that Manning (and/or assistant/associate coaches on his staffs at Tulsa and Wake) does a good job of developing big men. Who is responsible for what, though, is grounded in an assumption that Manning's coaching explains their success.
Yes, when you only look at the positive examples it is pretty easy to conclude that everything is great.