So the dismissal reads like "there's no excuse for not doing an extremely difficult thing that no one knows how to do reliably." I don't find that to be a useful or informative comment, so I agree with dang's approach here.
We already know from earlier twin studies that genetics play a large role in literacy and numeracy.[0][1] So it's not like this is a shot in the dark, everyone already knows it's a confounder that must be taken into account.
[0] https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-7610.... [1] http://journals.sagepub.com/doi/abs/10.1177/0956797613486982
On the OP getting warned, it wasn't immediately clear to me if the objection was more about a) knowledge that Matthew Yglesias didn't read the study and therefore was not in a position to summarize his analysis with a pithy analogy and be quoted, b) the use of any pithy analogy itself, or c) the use of an analogy that seemed to explain a confounding issue? There seemed to be some additional, non-obvious context missing (at least to me) that led to the warning.