You're right, our fear of snakes is baked in the hardware, I'm certain of that, it's an evolved recognition of danger that likely goes back millions of years. That's why my innate reflexes automatically kicked into action when I stood on the Eastern Brown.
I don't think I've ever moved so fast in my life. I was 14 at the time so I was light, fit and agile. That said, my reaction time would likely have been slower than the snake if it had decided to strike. That was decades ago before decent antivenin and I was a reasonable distance from medical help and also I was by myself (trekking in the Blue Mountains by oneself at any age is stupid thing to do, so I was very lucky).
With my snake encounter at Emmagen Creek things were a little different, I was older about 30 and I'd learned the worst thing to do was to startle the snake so one should keep perfectly still—and I did although it was difficult.
I still wonder about that encounter, the snake definitely† wasn't there when I arrived at the Creek's bank so I was moving about and making noise. Most snakes I've encountered whilst bushwalking I've seen at a distance—2, 3 metres away—and they quickly scurry off but this one came awfully close whilst I was moving around and it showed no sign of aggression or seemingly any concern for my presence.
† My grandmother came from outback NSW and was bush and snake wise so I was taught from a very early age to always keep an eye out for them whilst wandering through the bush and I did so at Emmagen Creek as I do when bushwalking anywhere. The fact is the snake arrived after me and I find that strange.