Several black holes have already been identified with masses of this magnitude. However, it seems that the age of this black hole is indeed problematic, as indicated by the article:

> For the black hole to grow to such a staggering size in less than a billion years, the astronomers posit, it must have been pulling in interstellar mass from its surroundings at the maximum rate the whole time. Even so, the radiation of the quasar formed by the black hole should have started to limit that mass accumulation before such a size was reached.

To which "accepted model" are you referring – the cosmological model for the Universe, the determination of black hole masses, the formation and growth of supermassive black holes, or something else? As @nilkn notes, this discovery is in conflict with existing models of the formation and growth of (some) supermassive black holes, which is a reason this paper is in Nature.