It actually has a pretty clear meaning: 99.9% means that someone ignorant (or lacking integrity) wants to project unrealistic certainty. That's what that phrase communicates.

Yeah, but it's not 99.9%, it's "almost 99.9%". Very weird statement.

I think Musk has said he'll get to 99.9999% safety by the end of 2020 (what he said they needed for complex intersections required for the level 5 robo taxi network they were to launch in 2020).

Yeah, it's a Yogi-ism. It's a sign of our time though, isn't it? Expressing absolute certainty is almost sacrilegious these days. "I'm 9.99999...% sure."

It means anywhere from 0% to 99.9% sure.

Sure...

Maybe they're insinuating that the position of the bodies rule out a scenario where the driver was thrown into the passenger or back seat via impact.

In a frontal collision unrestrained backseat passengers get thrown to the front of the car. Basic physics!

That the series of their surety converges to 99.9%?

https://en.m.wikipedia.org/wiki/Almost_convergent_sequence

My grasp of this advanced math relinquished long ago so I'm still not sure what it means.