YARN
Fleet Captain
This line never bothered me at all for the simple fact that this was stated/felt/shared during a mind-meld and is most likely hyperbole.
It's a mind-meld, not a hyperbole-meld.
Spock mind-melds Valeris in ST-VI precisely because he wants to cut through the BS and know precisely what she knows.
In this scene, Spock is telling Kirk (directly) what he needs him to know so that he can understand why old-Spock is there and why Nero wants revenge.
Of course, I also don't buy the argument from some that Spock would not use hyperbole. Even by STVI Spock specifically stated that logic was only the beginning of wisdom and the older Spock got, the more he seemed to embrace both his logic and his more emotional, human side.
Dude, the narrative point of the scene is exposition. It's a way to clue the audience in on the action. The destruction of the whole galaxy is a way to establish the stakes motivating Starfleet and the Vulcan Science Academy.
And why would he be "using" hyperbole on Kirk with regard to this particular point? If there were some "twist" at the end of the film, we might re-evaluate Spock to be a variety of "unreliable narrator" in this scene. But there isn't.
Destroying Romulus could indeed have galaxy wide, or at least Alpha Quadrant wide ramifications. Plus, being in a mind meld, Spock may not have all of his emotional "filters" up.
This is unadulterated horseshit.
The filmmakers wanted us to believe that the galaxy in the future time-line was in direct jeopardy from the supernova (and that globs of magic red goo create black holes). Mission accomplished. They successfully communicated what they intended to communicate.
If you want to speculate wildly beyond what we are shown on the screen, feel free -- but this is still apologetic nonsense to cover what you realize is a rather glaring science gaff on the part of the writers.