The accident rate comparison wasn't between miles driven by humans vs. miles driven by technology, it was between miles driven by cars before and after the technology was made available, and the accident rate went down. The only way that happens is if it does better than humans at the thing it's actually being used for. Which means that even if it's only used in clear conditions, it's doing better than humans did in clear conditions -- not just better than humans did on average.

It's plausible that it currently does worse with adverse weather than humans do with adverse weather, but I'm not aware of any data on that one way or the other, and I wouldn't expect any since it's not currently intended to be used in those conditions.