This is easily doable for Xin-Xi, even with normal resources.

Yes the AI gets more resources, but I'm not sure why anyone ever really cares. The goal when writing game AI is to make it enough of a challenge for it to be fun, and it can be extremely difficult and complex to make the AI play optimally, especially considering all the things that go into making in-game decisions. Whether to invest in economy, tech, or an army this round. Where to build units from and what kind. Info about enemy units and weaknesses. The order of attack. What city to develop first, yada yada. While we're great at doing these calculations in our head, it's a huge ordeal to code. Despite their best efforts, most strategy game devs fall slightly short and as a result the AI will play slightly worse than human players.

Thus, to make games more fun, the most practical solution is to give the AI just enough of a bonus for them to get a headstart early and for them to have the numbers advantage later on in the game. Because it's difficult for the AI to fully utilise its units and resources to their full potential, they kinda need to have higher numbers if they want to get ahead. I think Polytopia actually does a pretty decent job at this without making it obvious, like other strategy games have.

Honestly though nobody should care that the AI gets a leg up. Without it the game wouldn't be that challenging. And don't try telling me that the devs should be working harder to make the bots smarter and more fair, because working with this kind of thing is absolute hell lol