Imagine your dragon after one hundred turns of killing one high level power house unit after another.
Dragons don't exactly have an xp shortage, they have an opponent shortage. If you get the dragon as sheaim or the kuriotates, you're way past ending the game. It comes so late that you've probably already won by a mile anyways, and then you can start killing just about everything your opponents have at 100% odds without even using spells to damage them first.
The problem with the cult is it's not useful beyond the dragon. Earlier, it was founded sooner, spread around, nuked some culture production, made the founding city basically impossible to conquer, and let you build that dragon later. Once in a blue moon it would flip an enemy unit to your side, I never had it happen. With it being founded on building the dragon, it's just that much more unimportant. It never really has any bearing on the game as a religion, there are no religious units. The dragon itself in no way needs any help gaining experience, it's a game ender unto itself without any help at all if you can avoid getting mobbed bad.