It is possible that at some point, maybe 15 years ago, AMD provided sufficient documentation to write drivers, but even 10 years ago, a lot of documentation was

It is possible that at some point, maybe 15 years ago, AMD provided sufficient documentation to write drivers, but even 10 years ago, a lot of documentation was missing (without even mentioning that fact), which made trying to contribute rather frustrating. Not too bad, because as you said, they had a (smallish) number of employees working on the open drivers by then.