I think he’s getting at the idea that we’re losing some of the natural variability that used to characterize our climate. Sure, the upside (or downside, depending) is that we get a longer summer and more warmth, but that comes with a tradeoff. If the baseline keeps shifting warmer, it can have broader consequences down the road on ecosystems, agriculture, water resources, the frequency of extreme heat, etc. So an extended summer might sound nice for some, but the underlying reason for it is arguably a lot more concerning, and what it means in the long run.
Looks like Chicago is headed for yet another above average September, perhaps even well above average. 2014 was the last time ORD recorded a cooler than average September. Since then, literally every September has finished above average, with 2020 being the lone exception to that above average streak finishing exactly at average against our warmest 30-year averages. Pretty wild, and perhaps a bit concerning.