Yeah it's like 90% luck. There was this weird thing in the first half where because Morel was so comically unlucky, it took up all the air in the room and it was hard to have the conversation about Swanson without it coming off as excuse making. And forget about Miguel Amaya getting in on that convo.
And honestly I think the other part might be simple strength of schedule. There was a stretch in late April to early June where the Cubs played a brutal gauntlet of pitchers. Even teams that don't have the best pitching (the Reds for instance) the Cubs seemed to always draw the top of their order. By Pitch Info, in the first half of the year, the Cubs faced an average fastball of 95.01 MPH. The Braves were second at 94.68. That 0.33 difference is the same as the difference between the #2 Braves and the Orioles at #19. Velocity is not a perfect proxy for pitcher quality, but when you're more than a full standard deviation beyond your next closest peer it's a problem. Here in the 2nd half, Cubs are 27th.
Swanson has improved across the board in the second half, but when looking at peripherals rather than production, it's all small incremental improvements. He cut his groundball rate, but not dramatically. His K's have gone down a lot, but the contact/discipline numbers have moved improved far less. I think those peripheral improvements are the ebbs and flows of quality of competition, while the topline production is largely BABIP.