Don’t know for sure about the newer engines, but on older engines, it is a pedal that requires measurable force to depress. I believe that some of the european engines use a hand grip with a similar provision.
I doubt that the deadman switch played into this. It’s more likely that he was just not paying attention, and that the curve of the track kept the trains out of each other’s view until it was beyond the point of prevention. Trains take a long time to slow.
The conductor reported that the engineer missed the final two calls, thus the question arises as to whether the engineer might have been unconscious.