As I understand it, the DRM authenticates the physical monitor/TV with a key on the hardware. That part is uncrackable because authentication is done on Netflix

As I understand it, the DRM authenticates the physical monitor/TV with a key on the hardware. That part is uncrackable because authentication is done on Netflix-side. But as for whatever is rejecting Linux but allowing Windows/macOS, that has to be completely in-software. I think you are right--this could be bypassed by changing a register value somewhere. This sounds like a fun project.