That was not addressed in the film. According to Nero's own words in the movie, "In my time, where I come from, this is a simple mining vessel". Countdown doesn't count. Average moviegoers shouldn't have to read it to get that info. In the movie, it was a Romulan mining ship. That's all.
Of course, it's a "simple mining vessel". But Nero is also insane, and has been stewing in his insanity for 25 years. Besides, remember all that technobabble exposition that you guys want to explain x, y, or z?
This would be just more of that. No need to explain this "simple mining ship", since the people who would get twisted about it don't amount to much in numbers.
But, for the record, the "unofficial" explanation WAS written by the guys who also wrote the script for this film.
One is free to accept it or not. I accept it. Don't care if it's "teh canon". It makes sense.
The end.
Case in point, you want, for whatever reason, Warped9 to buy into an idea that SpockPrime isn't even "our" Spock, because of clues YOU have put together in the film, in defiance of anything the writers have had to say.
So...let's not pretend that you are content to just go along with what the writers "officially" intended.
It does NOT make sense for a mining ship to be so pimped out, so there must be an explanation. But then, I'm a lifelong Trek fan who knows something about ships in this setting. The mainstream filmgoer doesn't care and isn't going to need all the technobabble.