The reason they couldn't use LIDAR on the Tesla is because Tesla is literally the coolest car on the road, and the LIDAR protrusion on the top of the car would completely reverse that. It would go from the coolest car to the dorkiest car.

There isn't a way to just stick the LIDAR somewhere unobtrusive. So this is the thing that, strangely, almost no one (aside from myself) seems to have noticed: LIDAR is fundamentally incompatible with the current Tesla brand. And that was especially the case when there was only really the super high-end 'cool' model of Tesla vehicle. But still, the way the vehicles are marketed and designed today, they cannot add a functional LIDAR without that fundamental change.

This is a little bit ridiculous, but the reality is that at some level, most average adult consumers out there are not really much more sophisticated than the average kid in your 7th grade English class. So it just comes down to the fact that the thing on the top doesn't look cool.

Now, Elon Musk (and most Tesla fans) will never admit that.

You could add it as a fin, like BMW antennas. LIDAR is expensive and mechanically unreliable: you can't steer light electrically, so you need either a spinning motor or a phased array (under research). And isn't really needed after optical, radar and sonar.

I don't understand how it would look stationary. The car is moving towards the truck, therefore it would not appear stationary on radar, right?

Known car velocity is subtracted out. The bigger issue is why it wouldn't halt if a stationary object is in your lane. My guess is the sensors had too narrow a vertical FOV to detect a high tractor trailer.