Well, we have different concepts of doubt. Also, he's talking about proper procedure masks (ASTM Level 1), not paper or cloth or home made masks. These are hard to come by, and in any case should be reserved for high risk situations. There are no studies that I can find showing woven fibre or paper mask effectiveness, especially in regards to the risk increased contact transmission from touching the mask.
You can make the argument that homemade masks won't reach those levels of effectiveness because of a lack of a strong seal around the airways. And there have been no real-world case studies of cotton masks protecting people from getting sick.
But there are plenty of arguments to be made that support the idea that if 100% of the population wore a cotton shirt around their face, then the pandemic would be strongly mitigated.
His prior post has already proven this beyond a shadow of a doubt.
https://slatestarcodex.com/2020/03/23/face-masks-much-more-t...
Perhaps that post is a prerequisite to this post.