Aha - Seems to me proof that there are galaxies much further than the observable universe. In other words, it's bigger than we freakin' thought. That this event happened a long time ago, early in the accepted age of the universe, and we are just now seeing it is also quite mind-blowing.
I don't know about anyone else, but just reading the phrase "astronomers detected hundreds of galaxy clusters " was enough to stop me in my boots. Wrap your mind around that idea, then read the rest of the article.