This is a great point. I think the research is "biased" to begin with, in that it feels like you could easily p-hack your way into a "chat gpt has degraded X" kind of paper if that's what you want to show. But with SO in particular, you're right that there are actually clear improvements that it makes as a competitor.

I can't think of a way, buy it would be interesting to see if there is a similar effect on Wikipedia contributions.