If I understand it, the mind does not look closely at every line of every letter but reads it in outline. So a word is not much more cognitive load than an icon.
There is no learning curve, as with icons. With icons, the problem is not recognizing what the icon looks like (an envelope!) but what it means within the application (I'm looking at you, GMail).
Selfishly, as a developer, text labels are easier than a folder full of image assets. But I don't feel bad about it, because it's better for the user too.
DISCLAIMER: These are not absolutes. There are exceptions. The article's tack is reasonable. I'm saying that icons are overused in general.