ATI/AMD open source linux support has been blowing hot and cold for over 25 years now.

They were one of the first to actually support open source drivers, with the r128 and original radeon (r100) drivers. Then went radio silence for the next few years, though the community used that as a baseline to support the next few generations (r100 to r500).

Then they reemerged with actually providing documentation for their Radeon HD series (r600 and r700), and some development resources but limited - and often at odds with the community-run equivalents at the time (lots of parallel development with things like the "radeonhd" driver and disagreements on how much they should rely on their "atombios" card firmware).

That "moderate" level of involvement continued for years, releasing documentation and some initial code for the GCN cards, but it felt like beyond the initial code drops most of the continuing work was more community-run.

Then only relatively recently (the last ~10 years) have they started putting actual engineering effort into things again, with AMDGPU and the majority of mesa changes now being paid for by AMD (or Valve, which is "AMD by proxy" really as you can guarantee every $ they spend on an engineer is $ less they pay to AMD).

So hopefully that's a trend you can actually rely on now, but I've been watching too long to think that can't change on a dime.

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.