It's a joke (meme, copypasta) but it's also accurate.

As I understood: Missile uses gyros and accelerometers to figure out how far it flew already towards the target. This is not 100% accurate.

So it additionally uses terrain mapping to figure out how it looks down there. Compare ground (am I flying above a slope?) to its internal maps and it can figure out where it is and adjust path if off course.

So it figured out the position by knowing that the position is most likely not the position where it's supposed to be.