It seems like a shame, but it's unclear whether Ewing actually had a marked impact on Dwight or Yao. Seems like Olajuwon was the better tutor in both cases.
Olajuwon was always the more creative post player and the better passer. Ewing never had a penchant for looking up and making the high-post pass.
Ewing effectively has 2 strikes against him:
* he never really seemed to see the whole floor as a player
* superstars rarely make good coaches, presumably because they have some innate talent or understanding of the game that the normal player cannot easily grasp. (ie Just because Magic Johnson "tutors" you, doesn't mean you'll ever get his court vision, speed, athleticism, long arms, reaction time)
What they can teach you is their approach. In Olajuwon's case, he teaches footwork, he teaches you how to dance his step to get close to the basket. From that point on, it's your job to make the layup or short jumper. He'll also give you a few counter moves.
Not sure what Ewing did. Statistically, Ewing was roughly as effective as Hakeem, but the way he went about his business was entirely different, but less artistry, much more blue collar. (one can say similar things about Abdul Jabbar - he didn't have a huge repertoire of moves, but that sky hook was damned effective... and hey, everyone would be shooting it that way if it were something easy to master - but that has to do with his innate feel for the ball and the basket)
There's part of footwork that can be learned, but there's something (a combination of balance and agility, I suppose) that let Hakeem own it like few players can. (make no mistake, there's good reason why athletes like Kobe/LBJ could get something out of it - because they too are that gifted)