Seems to me that it's the typical case of turning left without making sure that cars on the lanes you're crossing will not need to use the brakes to not hit you. This happens constantly everyday, yet the only consequence is some cursing from the people that need to use the brakes, and that's it. However, in this case, Tesla vehicle didn't have anyone to hit the pedal...
So, both are to blame. I'm sure Joshua would have avoided the accident if he was paying attention. However, the truck's driver committed the biggest negligence, in my view.