Of course, McArdle's claims about what people would have said before the study, if asked, are also only being made after the results are known, which as you say is a classic failure mode.

I don't think that's true; if you read her original article on the subject, linked in the one I link, she quotes statistics like this:

Most of you probably have probably heard the statistic that being uninsured kills 18,000 people a year. Or maybe it's 27,000. Those figures come from an Institute of Medicine report (later updated by the Urban Institute) that was drawn from [nonrandom observational] studies.

And back in 2010, she said

I took a keen interest when, at the fervid climax of the health-care debate in mid-December, a Washington Post blogger, Ezra Klein, declared that Senator Joseph Lieberman, by refusing to vote for a bill with a public option, was apparently “willing to cause the deaths of hundreds of thousands” of uninsured people in order to punish the progressives who had opposed his reelection in 2006. In the ensuing blogstorm, conservatives condemned Klein’s “venomous smear,” while liberals solemnly debated the circumstances under which one may properly accuse one’s opponents of mass murder.

I don't think her statement is entirely post-hoc.