[balfirevic]

> Am I missing something here? Everyone seems thoughorly convinced that this is perfectly normal.

What you're missing is that one of the intuitions you have is simply wrong. The intuition is that sample size can undo the ill effects of non-random sample. As stated in the original article and elsewhere in the comments, it cannot:

> It is an error to use the sample size of a non-random sample to support the underlying comparison with the population of interest. Sample size can decrease random error, but not bias