The main point of the article, as I see it, is that the "em" used in fonts isn't the intuitively correct em size. It's just a control box used by the font deve

The main point of the article, as I see it, is that the "em" used in fonts isn't the intuitively correct em size. It's just a control box used by the font developer, and the result is that designers can't get a consistent font size. They can know, more or less, what something is going to look like given the font they want to load, but the fallback font might have a different concept of em size and look more different than it would need to be. So having a different metric, which is exactly capital height, makes fonts look more similarly-sized to each other at the same capital-height resolution, which he demonstrates in the article. This seems valuable to me.