In terms of (2), I wonder if it's even possible to write a driver without such a document. In the end, the vendor is on the foot for the driver for major platforms (let's assume Linux) - if they can write a Linux driver without a similar spec to this doc, then the doc probably doesn't need to exist since the business wins from hobbyist drivers will be low. If they can't though, then it's just a matter of formatting an internal document for public consumption - the doc itself has to be maintained anyways so the cost seems lower and maybe reasonable. I have a feeling the doc is necessary but I am not specialized in the field.

Assumptions, fair or not, about (1) seems more likely somehow.

Didn't all the asahi Linux Mac m1 drivers essentially get reverse engineered with little to no support from apple and no public docs? If I'm remembering correctly then I guess it's possible with enough effort and reverse engineering skills