I observed that the AI began to gather stone in the arena just as I did.
It was unable to see me taking stone from within the boundaries of my base, which makes me believe that it was cheating.
I’m almost certain that the AI wouldn’t have collected stone if I hadn’t been doing so.
Additionally, it delayed advancing to the Castle Age by reassigning two gold miners to gather stone, which was a strange decision and makes me think that it was being forced to do so by an external trigger.
This leads me to wonder if there are any other cheats the AI uses.