A. "The rest is history" is a common phrase. It is used when you are explaining the origin of something famous, and you have reached the famous part that everyone already knows.
"Do you know about the origin of tea? A scholar was sitting beneath a tea bush boiling water, a leaf fell in, he drank the water and felt energized, and the rest is history."
B. "X is history" is a completely different expression. Here, we are saying that X is useless and outdated, or that we have rejected it.
"Do you have any cassette tapes?"
"What? No! They're history! I threw away all of my cassette tapes a long time ago!"
Here, the writer is using structure B, but it sounds like (s)he is using structure A. So, it is a pun.