If this is the state-of-art then I have some bad news. I recently finished writing my thesis and copy-pasted several paragraphs that did not included any mathematical formulas. Most of them were classified as written by a machine (8 out of 12). It might be due that I am not a native speaker. It seems that the model struggled the most where there are mathematical details discussed but on the topics where I wrote more freely as conclusions and some analysis, it classified as written by human.