Some assorted thoughts:
You weren't 'discounting' the first two months. You were ignoring them entirely with all the 84 win team talk (I've got 45-40 translating to 86 win team, but splitting hairs). Ignoring them because they were so historically good makes even less sense. They showed elite levels of production for two months. That absolutely holds some weight.
If you had some all powerful/all knowing deity come down and tell you that this team was a true talent 84 win team, and then you told me that since June 1, they would have by far the biggest disparity, in a negative direction, between their xwOBA and their wOBA (no other team is above 20 points, we're at 27)....I would expect them to play a lot worse than at an 84 win pace? I mean, you could also argue that the statistic as a concept is some degree of flawed/worthless, obviously a different conversation, but setting that aside the Cubs are a pretty extreme outlier. Now you could argue we've been on the other side of that luck on the pitching side but I don't think there's any way to say it's been overall equal. And I've been very negative/critical of the pitching staff and the way it was assembled/wasn't supplemented at the TDL. But I also expected Boyd or Horton to go on the shelf by now, and every day is another day closer to the playoffs with them still penciled in for 2 of the 3 first round starts. We all bemoaned the concept of having to start Matt Shaw every day and not getting Suarez, and we've all seen what's happened since. Hitting can turn quickly.
I'm sorry but the other teams still matter. Cubs are 45-40 since June 1. We're very likely to play the Padres in the first round, probably in a three game series all at Wrigley. The Padres are 46-41 since June 1. The Dodgers are 43-42 in that stretch. Theoretically by that measure they also 'don't have a legitimate reason to turn around'. The Brewers, historical stretch, discount/toss that out, 11-10 since the start of the 5 game Cubs series. I guess Phillies will just cakewalk? Because they've played 2.5 games better than us over a 2ish month stretch? The 2025 Cubs don't have to beat the 2016 Cubs to have success in the playoffs. They need to beat three or four similarly flawed teams in what are glorified coin flips.
I get your point on the sample size, and I'm very aware 9 games or whatever is dumb. It was mostly reactionary in that every. single. loss becomes this referendum of 'see, I told you they were bad'. Every loss is 'horsefeathers pathetic' etc etc etc. But we've yet to have a truly bad stretch, which is something you can't say about any of the other teams. 90 win teams don't win 55% of their games every week. They get hot, they play mediocre, and ultimately that's where they end up.