I think the concern is that the Tesla system may not have even identified the semi trailer across the lane as an obstacle at all because it is not clear that the Tesla's brakes were applied prior to the collision. Any defensive human driver would have taken note of the truck in the turn lane before it even started its turn and prepared to take action if it turned unexpectedly, even if they were unable to stop or swerve in the end. Tesla's system was definitely unaware of the truck until it entered the lane, and was most likely incapable of determining the actual structure and size of the obstacle.
But the big problem everyone is worried about is right there in the first sentence of your link. The article calls it a "self-driving car" which even Tesla (except for their CEO in promotion mode) takes pains to say it is NOT. Treating a Tesla on autopilot as a self-driving car is extremely dangerous to the driver and all motorists around them. The sensor package and software is not designed for that usage.