Why can't people just say whether they use aggressive AI or not
without adding the remark that their setting is a more difficult game setting and causes a more challenging game.
The remark that I think was insulting went further than that by the way:
Aggressive AI is the way it is meant to be played, not having it on is for more casual play in my view.
Calling someone else's game settings useful just for more casual play is insulting, calling someone else's game settings the incorrect game settings is pushing someone else down so that you yourself arrive on the top, it's elitism and it's just plain unnecessary.
What results in a more challenging game for someone depends mostly on how that player plays the game.
I myself like to use the military aspect of the game when it is to my advantage. I'm not really a rush-player as I play on huge maps that aren't overcrowded with AI and thus the rush is not that effective. But I do use the military aspect when I can take advantage of it. The normal AI is not that effective in attacking other players or defending itself. The number of units needed to crush it, makes war in general a fairly efficient way of improving your civilisation. The aggressive AI makes war far less efficient and can pose a military threat to your civilisation. I like that. For me that leads to a more challenging game because I can't exploit the military weakness of the AI (as easily).
However, if you like the builder part of the game more and don't like the militaristic part of the game very much, if you aren't rushing the AI and aren't taking advantage of every military weakness that you see, then you will probably find a more challenging and fun adversary in the normal AI. This AI will not invest in the huge armies it needs to defend itself against an aggressive human player and thus will be better in the technological development of its civilisation. It can challenge you more in a pure space race game without extensive military conquest. This AI will let the human player develop his/her empire without using every weakness it sees to declare war. Many players prefer that setting. Against a human player capable at defending itself and not interested in military conquest, this setting will probably be more challenging (and fun).
Now if you start to think about what would be a more challenging AI opponent to a theoretical optimal playing human player, a player who uses every loophole to win the game and knows every strategic trick to win the game, then I would think the answer to that question would be very map dependent. The answer would be different on an overcrowded pangea map compared to a isolated start on an archipellago map with no contact between players until astronomy.
Even if you would arrive at an answer to that question, it wouldn't be an answer to the question how the game was 'meant to be played'. Game theory will never answer such a question. It just tries to discover the optimal strategy to win the game, not the 'way the game is meant to be played'. That last question is a philosophical one and while I do believe in mathematics, mathematical game theory can't answer that question.