Mining operations are obviously quite secretive about any edge they have againstb their competitors but its not much of a stretch when there are stories like this one: https://videocardz.com/newz/zotac-publically-endorsing-gpu-m...

This doesn't seem that far fetched. Especially since miners don't need to produce full-featured drivers.

When GPUs are know to use signed firmware, when open source coders need to ask for signed firmware to make even a minimal feature open source GPU driver work, and when there’s no evidence at all that high performance custom made mining-only drivers exist, then that seems very far fetched to me.

Not so much a new driver, but here is an example of drivers being patched for mining. https://github.com/keylase/nvidia-patch

Indeed. Not a new driver at all.

What this tells me is that they just didn’t care enough to lock this down hard.