It felt like more than a few inches. (I'm not splitting hairs here, I really do feel it was qualitatively significantly different from your description, which made it sound like the car was cautiously starting to move just a bit.)
> and Elon immediately put on the brakes. The situation was no more dangerous than when a human driver mistakes which light is theirs and does the same thing, which happens pretty regularly in my experience.
When humans make this mistake, they stop the car themselves. When this car made this mistake, someone else (Elon) had to stop it. These are not equivalent. In the former case you can argue no accident was going to happen, but in the latter you can't.
> that this is a test version of the software isn't irrelevant, it makes a huge difference—I am much less opposed to internal company testers who know what they're doing
This is on a public road. Try pulling this off (well, please don't) in medicine and see how it goes.