Yes, I agree with that. But that has nothing to do with AI improvements. It's true now, and it will be just as true (no more and no less) if the AI improves.
But, in my experience, the gap
has become more pronounced as a consequence of both the added advantages to warmongers in Warlords, and the improved AI with the last patch.
It seems to me that if there are new military features to exploit (I don't necessarily mean 'abuse'), then the human gets the best of it, provided they take a warlike approach. At the same time, if the AI gets better at building, the increased difficulty weighs more heavily on the builderish player, since they have to compete directly with the improved AI, whilst the warmongers are merely faced with slightly better developed enemies, but with the compensation of conquering better developed cities.
Perhaps, it might get to be slightly less true, if the AI gets better at defending itself
That would indeed be a major improvement. The main problem, imo, is the AI being so completely incompetent at defending its cities, with the result that warfare represents the single greatest advantage a human player has (a situation which has been made even more pronounced by the AI's improved building powers).
However, I think some extra bonus (maybe a diplomatic one) for those that take a peaceful approach, or an extra handicap for those who pursue a course of relentless aggression, would also help to balance the game better.
For example, and returning to the issue which sparked this debate (iirc

), perhaps the WFYABTA limits could be increased if the player consistently shuns warfare, or reduced if the player frequently starts wars with other civs. If a player demonstrates a propensity for endless backstabbing and violence, then maybe the AI civs could simply refuse to trade with him/her (which would be an entirely rational decision, since that player clearly represents a threat to anyone and everyone).