I'm surprised that it's not very good. I'm working on an MCP for a Foss civ 5 reimplementation (unciv) and I find that 5.6 Luna is better than the built in AI, and costs about $2-4/player/game at API prices. I suspect Astra would blow humans out of the water, but I don't have $1k/player/game to spend on that lol
Maybe civ 5 is more reactionary than strategic, so simple strategy that doesn't look too much into future works reasonably well. Or maybe civ is more mainstream and models absorbed more knowledge about strategies that work reasonably well.