That’s cute, but not true. The author mixed up px units with the reference pixel definition. A px unit is defined by CSS to be 1/96th of an inch, it is not an angular measurement.
What is true is that if you make a CSS moon 24.3 px wide and display it on a page with no zoom, on a monitor with 96 device pixels per inch, and view it from 28 inches away, then the size of the moon on your screen should be approximately the same size as the moon in the sky.
What is true is that if you make a CSS moon 24.3 px wide and display it on a page with no zoom, on a monitor with 96 device pixels per inch, and view it from 28 inches away, then the size of the moon on your screen should be approximately the same size as the moon in the sky.