I’m assuming nothing. Maybe it braked, maybe it didn’t. Maybe it had an opportunity to brake, maybe it didn’t. We don’t know, and frankly it doesn’t matter. What we know in this instance is that whatever went down the human backup didn’t spot it in time either. Which happens. We live in an imperfect world. You want to make assumptions and play the luddite, but out here in reality all the current data says a human wouldn’t have done any better (because they didn’t).
That's also an assumption. It takes 145' to 195' to stop from 45MPH (depending on reaction time, weight of vehicle, driving conditions). It was dark, which can be a delay in itself if whatever is out there isn't in the headlights (shouldn't matter to a vehicle with all kinds of sensors).
All we know is someone is dead, and the death was caused by a collision with an Uber vehicle that was in autonomous drive mode with a human as backup.
Don't take this as an endorsement of autonomous vehicles. I don't want them on the roads with me.