Ignoring for the moment the accuracy of that tool, even if the prose is 99% AI written, that doesn't meant that the result is fake. Maybe their model wrote it.

Edit: You can try it here https://huggingface.co/spaces/THUDM/GLM-130B. I tried it. I was not impressed.

Maybe I'm doing it wrong but I pasted the article text into the site you linked and it said 99.98% real.