That's not completely true.

https://github.com/commaai/openpilot/blob/master/docs/SAFETY...

It's an... interesting approach, They essentially reduce the surface area as much as possible. I don't buy that it's enough, but, again, interesting to see what they do.

Besides, a big OEM could pour an army of developers and turn the Comma approach into an ASIL D, it would be quite a lot of work but within the realm of possibility.