Something feels off / sloppy with the article in general and that’s gotta be the smoking gun. I cannot think of a single case where this is true, I’ve lost 35 pounds in the last 6 months, and I’m still a medium. 5’9”.

This rule of thumb, when used correctly, is supposed to refer to womens' dress straight sizes. As in, for an average height woman, losing 10 lbs is roughly equivalent to going from a size 8 to a size 6. It doesn't refer to clothing that is sized as "medium", "large", etc. (I did not read the article; it seems from the quote that the author was using this rule of thumb inappropriately and out of context.)