While this advice is, of course, true, it's also pretty common for introverts without a lot of social experience to not notice when they're making someone uncomfortable. So, starting conversations is a great way to get over the fear of them, ending them in a reasonable time frame is a great way to insure you aren't crossing the boundary into uncomfortable (for the other person...I know that the introvert is going to be uncomfortable throughout the exchange, as I'm an introvert myself, and so the cues that may be obvious to most folks aren't as obvious to an introvert...insert boiling frog analogy here).
We just don't get energized from being around people. This doesn't mean we can't or don't. Like any aspect of temperament, it's mostly immutable: even a socially adept introvert (which I like to think I am) is still an introvert.
Wikipedia puts it well: "Introversion is not the same as shyness, though introverts may also be shy. Introverts choose solitary over social activities by preference, whereas shy people avoid social encounters out of fear."
He later told me that he didn't feel at all comfortable during the conversation. He said that he has been trying to "bust out of his geeky shell" by talking to random people. To me it looked like he was just a friendly guy chatting with a stranger.
Moral of the story: just do it. It will probably feel weird but you are the only person who will notice.