Why you should always assume the driver has not seen you

Having a right of way and speed for the circumstances are two different things.

Just the same for a car driver approaching the same situation, they should not drive straight across a roundabout at full speed either.

but full speed for cars is a very different thing to full speed for bikes. Roundabouts work best when all vehicles move at similar speeds so it's often safest for bicycles to sprint onto them. Yes, in this particular case we can see he could have avoided the accident by slowing down on approach to the roundabout, but only with hindsight do we know that. The car ahead that took the route he was taking was probably going just as fast.