That really looks like a terribly small dataset on the "before autosteer" side. According to footnote 21, we are looking exclusively at the sum of miles Teslas had on the clock when someone paid for the post-factory activation. Due to the growth of Tesla sales, that set is likely biased quite a bit towards younger cars with few miles at time of update availability. Tens of millions of miles might sound impressive for a statistical base, but tens of airbag activations does not.
At least they did not compare "all Teslas" vs "Teslas with Autopilot Technology Package checkbox checked", since that would select for drivers who do a lot of miles in boring long distance driving. That kind of driving happens to be the one where any car has an "airbag mileage" far above average, so it would be just natural that they have a lower crash rate, autosteer or not.
But a part of that effect might still be present in the numbers if customers are less likely to pay for a post-factory update than while ordering the car (which I do suspect). If this holds, then the subset of drivers who activated post factory (with miles already on the clock, so that this part of their car usage appears in the "before autosteer" column) will have, on average, a bigger desire for autosteer (and more naturally safe long distance cruising in their driving patterns) than the average of those who got the car with the option factory-activated (and who thus appear only in the "after autosteer" column).
Edit: oh, and here's the really big one: the "before autosteer" number only includes cars that where upgraded. How is this important? All airbag activations that happened in an event that messed up the car beyond repair cannot appear in the "before update" dataset, because nobody spends thousands of dollars on an autopilot update for a car wreck.
[edit 2]: This should actually make the "before" number much better than the "after" number, so i really wonder what's going on here.