Looks like they got that number from this quote from another arstechnica article ”…OpenAI admitted that its AI Classifier was not "fully reliable," correctly identifying only 26 percent of AI-written text as "likely AI-written" and incorrectly labeling human-written works 9 percent of the time”

Seems like it mostly wasn’t confident enough to make a judgement, but 26% it correctly detected ai text and 9% incorrectly identified human text as ai text. It doesn’t tell us how often it labeled AI text as human text or how often it was just unsure.

EDIT: this article https://arstechnica.com/information-technology/2023/07/openai-discontinues-its-ai-writing-detector-due-to-low-rate-of-accuracy/

That may have been the goal. Look how good our AI is, even we can't tell if its output is human generated or not.