I'd never heard the term before and was thinking something completely different... Like maybe someone like me who, when I finally have a good chunk of time, pokes around on this site and reads random people's posts weeks and months after they were originally posted and then leave comments on them. Voila! A "Post Turtle"!
It's a political joke.
When you see a fence post with a turtle balanced on top, you know he didn't get up there by himself. He doesn't belong there, you wonder who put him there, and he can't get anything done while he's there.