The article is obviously using hyperbole. It's not a flop in the sense that it's losing money. I, who was disappointed with the movie, have no problem saying that the movie is making good money.
But I think it could well be a flop by the standards of "Will make more money than Ep1", which it has not done. It's going to make everyone money, certainly, but it's not going to beat Titanic, it's not going to beat Ep1, and it's not going to break the big box office slump that the entertainment rags keep complaining about.
By the lore of the first geeks, it was supposed to be the fantastic conclusion to a fantastic series that made a massive ton of money, more than any Star Wars movie had ever made, and won that Academy award for Best Picture that Annie Hall so unrighteously stole from "A New Hope" decades before. That's what it was supposed to be, and while others here on this board may feel that it was certainly that way for them, the box office has spoken differently.
That's not even wholly a judgment of RotS, although the critical "enh" I saw in most places (usually around a B- or 2-star level in most non-geek movie areas) certainly didn't help. An old author whose name I forget said something that I'll badly paraphrase with "The beginning of my book sells my book. The end of that book sells my NEXT book."
TPM and AotC hamstrung RotS, both by hurting the number of people who could have come and by lowering the overall enthusiasm such that people who did come weren't all jazzed and starry eyed and didn't overlook the flaws as readily and weren't willing to see it multiple times. (Yeah, I know that people here did. I get that. I know. Really. I'm speaking generally.) So you have a movie that did very respectably well, but isn't going to shatter the records.
And it really really really could have, with a stronger script, better fight choreography, and two lead-in movies that were, if not wonderful, at least not disincentives.
It didn't do as well as it could have, should have, would have. Which makes it, if you apply that standard, a flop.