That is knowing a specific environment and its tools very well, knowing what issues people tend to run into, and knowing how to present that information.
The initial knowledge is helpful while programming, but is only one needed skill of many that a programmer has to put together at the same time. The skill at exposition makes me look good, but that's a documentation skill, not directly a programming skill.
Furthermore that particular explanation is a variant on one that I've both seen given, and attempted to give, before. It is much easier to come up with a nice way to say something when already you've tried to say that thing a few times. Even if you don't clearly remember how you said it before, the experience guides you.
Cheers,
Ben