Number of data points aside, I would like to see one causal model worked out in depth. Do you have a sample link?

Actually, I believe the claim in the article is precisely that spending more time leads to better posts and that this leads to more subscribers. Basically, the author feels that he achieves more (in readership growth, but also on other dimensions he cares about) by taking the time to write one really good post instead of using that same time to write two mediocre posts; he contrasts this with the standard advice that publishing frequently is critical to building an audience.