I used to struggle with this a lot. In fact I never got into open source programming because I simply could not understand where to begin in understanding foreign code bases.
I still struggle but it's easier than it used to be, this is just something that comes with experience, and is the biggest 'tell' between junior and senior developers.
Heh, how did you find that old post? Interesting nevertheless.
I am right now at work, struggling to understand other peoples code. Simple google search led me there.