Musk may be right or wrong, but this article is a non-sequitur.
Except people don't drive reliably in inclement weather at all, so you don't really want that as the gold standard.
Training a car to be as good as average people driving in the rain/snow would be horrible.
So what was Musk's point ?
Isn't that pretty much what you said "Full self driving" does NOT imply?
“Full autonomy” (at least to me) is being able to do any trip. Not just some or most. Because the key benefit is that the car can be empty, or the passenger drunk/blind/...
That’s what I think the crucial difference is between their marketed “full self driving” and true full autonomy.