I think the quote is from Jim Manzi rather than Megan McArdle, given that McArdle starts the article with

I asked Jim Manzi, who has literally written the book on randomized controlled trials, to share his thoughts. Below is what he said:

and later on in the article it says

I agree with the weight and seriousness of each of these objections. My agreement is not ad hoc; I wrote a book that tried to describe how businesses have implemented experimental processes that operate in the face of all of these issues.

suggesting that the whole article after the first paragraph is a quote (or possibly paraphrase).