So, I would say that while I wouldn't fully dispute your claim that attributing authorship from text alone is impossible, it isn't yet totally clear one way or the other (to us, at least -- would welcome any outside research).
Long-term -- and that's long-term in AI years ;) -- gaze tracking and other biometric tracking will undoubtedly be necessary. At some point in the near future, many people will be wearing agents inside earbuds that are not obvious to the people around them. That will add another layer of complexity that we're aware of. Fundamentally, it's more about creating evidence than creating proof.
We want to give writers and students the means to create something more detailed than they would get from a chatbot out-of-the-box, so that mimicking the whole act of writing becomes more complicated.