Agree that comparing highway to mixed is not useful. But Tesla's data does show valid comparisons:
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Note the gray line at the bottom is US benchmark. Then red is no AP and no active safety, Teslas get into fewer accidents. That's kinda interesting. It implies Tesla owners are better drivers. And when you look at orange, where active safety features are enabled. Safety ticks up a bit.
So far, no AP involved, so if you think AP data is biased due to highway miles, then red/orange can be assumed to be a mix, since AP is out of the picture. Without AP but with active safety, Tesla is already 4x above the US benchmark.
If you consider Tesla's data set to be not large enough, just give it a year or two. At Tesla's growth rate of selling cars, it should become very apparent within a few years where AP safety stands. It won't be just early adopters driving Teslas anymore. Will we see the red line start to trend down? Will AP data on its own continue to trend up in safety?
Just calling out a flawed comparison between highway-biased miles vs mixed driving isn't sufficient to refute the safety benefits of the active safety features and AP. I would tend to agree with your cynicism if over time, AP safety trends downwards as the sample size grows. But that's not the case.