Oh ok! Got it. I read it as you saying ChatGPT doesn't use GPT 4. It's still unclear what they used for part of it because of the bit before the part you quoted:

For each of the 517 SO questions, the first two authors manually used the SO question’s title, body, and tags to form one question prompt3 and fed that to the Chat Interface [45] of ChatGPT.

It doesn't say if it's 4 or 3.5, but I'm going to assume 3.5. Anyway, in the end they got the same result for GPT 3.5 that it gets on HumanEval, which isn't anything interesting. Also, GPT 4 is much better, so I'm not really sure what the point is. Their stuff on the analysis of the language used in the questions was pretty interesting though.

Also, thanks for finding their mention of 3.5. I missed that in my skim through obviously.