Ah, excellent point.
Spock and Nero would still be at one and the same location when the supernova was tamed, but your argument makes it possible that this location might be something sensible like the center of the supernova explosion.
Then again, the mind meld visuals suggest red matter deployment from outside
the roaring wall of flame that was seen consuming Romulus. Are there perhaps multiple concentric walls of flame, and Spock strikes at the innermost one? That particular wall does show noticeable curvature - and a natural supernova would
feature concentric waves of destruction, possibly with the outer ones being less harmful (that is, gonna kill Romulus but only cause faint aurorae at Vulcan and not be felt at Earth at all) but the inner ones being something Spock must absolutely stop.