Simon's economics are strictly small-time while he is trying to sell a wagon of booze. He knows some economics and politics because he's spent 30 years trying to get allies against the Incubus King (and failing, but still). Megail starts handling economics pretty early. The game doesn't do a good job of showing you Janine's actions but she's not sitting in her bedroom eating sweets all day. And he knows theology partly because he's been involved with the Church for 40 years being friends with people like Sarai and Kara, partly because his late wife was a divine mage, partly because the Church (including but not limited to the Chosen) does have real power worth investigating and using even if he's not sure about the theology being correct.
Simon still impressed Megail enough with his understanding of economics for her to eventually give him a chance as a business partner, considering how blunt Megail is that wasn't exactly a small feat.
The game mentions a lot of stuff Janine does, but most of it is boots on the ground kind of stuff, she's a great public figure but the game doesn't show her being much of a mastermind, at least early on. She comes across to me as lacking initiative and the extent of her intrigue seems to be that she knew to keep the army loyal to avoid a definitive coup. I'm probably underselling her a bit, after all she was one of the people involved in creating the Doomed King plan and she gets better eventually, but I still hold the impression that Simon knew how to deal with Yhilini politics better than she did.
Simon hadn't seen Sarai in a long time before the start of the game, Wendis was a divine mage but she was very focused on the theory, so much that, being an atheist like Simon, her belief was that divine magic did not come from a higher power but was just as much of a natural part of the world as arcane and lust magic, Kara was surprised about Simon's theological knowledge at the start of the game, implying she did not talk about such things with Simon before. He having some knowledge is only natural, but considering his personal beliefs and the fact he stuck with the Chosen for so long as his best chance of defeating the Incubus King, you'd think he would have dismissed theology earlier and so he knows more about it than you'd expect.
And all of this while taking some years to raise a young daughter, and wasting a lot of years being, quite literally, stuck in a rut with the Chosen stuff. Nothing about Simon is, individually, impossible. But the combination of all of those things is nothing short of impressive. Drawing from my previous Ozymandias comparison, here's an excerpt from said character: