Per the article. The 26% is the chance an AI piece is labeled "very likely AI". So they can catch out over 1 in 4 pieces generated by AI. The majority of AI writing doesn't get caught, but this also means the vast majority (>99%) of non-AI work doesn't get labeled AI.

In the context of academic work. Universities are at very little risk of accusing a non-cheater of cheating. The 1in4 catch rate while low is a huge deterrent for potential cheaters. If I knew that I had a 1/4 chance of getting caught and punished, I would not cheat. Especially as i had to submit dozens of papers as part of my degree.