It’s linear output; the PQ is just the “down the wire” encoding, and the display undoes it back to linear photon light.

I don’t understand what you mean here. On the last engine I worked on, PQ encoding was done in a shader in the last rendering pass, which then ended up copied into the back buffer. That’s what I call output.

Did we miss a magical hardware feature somewhere so that we could have kept output linear and just let the hardware do its thing ?