The introvert/extrovert spectrum is an aspect of personality that seems to hold true across all cultures. Everywhere we've looked, there are some people that prefer to be by themselves more, and some people that prefer to be with other people more. The distribution of these people follow a standard bell curve. This is probably the single most well supported concept in the scientific literature on the topic of Personality Psychology. I'm dismayed that the author only made one reference in his blogpost -- a century old quote.

It's absolutely crazy that this exists across cultures and implies that the disposition is biological in nature. The term has its flaws, and its pop-psychology understanding is limiting. I do think the concept is pointing towards something universal about human psychology.