They used a difference-in-differences[1] to control for such longitudinal/temporal effects. Since ChatGPT isn't readily available in China or Russia, but StackOverflow is, they can compare how SO changed pre and post ChatGPT in countries where it is widely available and compare that to the pre and post in countries where it isn't available (basically as the control).

[1] https://en.wikipedia.org/wiki/Difference_in_differences

As someone who is getting their start with coding, where/what forums exist that have a high quality/helpful community? My biggest struggle has been with relatively simple questions – with a broad stroked theme/issue to 'em for the most part. AKA having a mentor or just a group of coders who are willing to help out if you are willing to be an active member (but I'm relatively useless, aka active maybe in an off-topic lounge part of it). I'd appreciate it if ya got the sauce, a PM if you'd like to keep it on the DL perhaps? Thanks!