I don't actually understand how Tesla autopilot detects cars ahead of it--but does this experiment properly simulate the stationary car, e.g. is it made of metal (and is that the material that Tesla autopilot is designed to detect)?
At the end of the day it doesn't matter if this experiment properly simulated a stationary car - unless you could argue that the Tesla was prescient enough to understand it was going to run into a car dummy. Any kind of road obstacle, be it construction, debris, or a wall, isn't necessarily going to look like a car.
No, the TL;DR is that all semi autonomous systems ignore stationary object as it is difficult to precisely measure if the object is in the lane. Highways have a ton of stationary object such as overhead signs, debris, construction stuff. Trying to accurately process all of these items will lead to frequent wrongful hard braking leading to rear ended accidents. That's why they are ignored.
And yet, Subaru EyeSight can detect and react to stationary objects without any of the problems you describe. Do they have some magic technology unavailable to Tesla?
Then there is no reason to market such systems and their imaginary future capabilities to be completely driverless.