This would naturally follow if the supernova were Romulan homestar blowing up. And this is exactly what the movie shows happening.
Okay Timo, I re-watched the scene, and your post is still confusing. According to Spock voice over ...
One hundred twenty-nine years from now, a star will explode, and threaten to destroy the galaxy. The star went supernova consuming everything in its path.
At this point the star has already gone supernova.
I promised the Romulans that I would save their planet.
After the star supernovas, Spock is on Romulas conferring with Romulan leaders.
We outfitted our fastest ship.
So Spock then traveled from Romulas to Vulcan, and a ship is outfitted
Using red matter, I would create a black hole, which would absorb the exploding star. I was en route ...
Now Spock is traveling from Vulcan to the supernova.
... when the unthinkable happened. The supernova destroyed Romulus.
And we see Romulas hit and broken up.
I had little time. I had to extract the red matter, and shoot it into the supernova.
Now Spock shoots the Red Goo into the supernova.
So, the star goes, then Spock has time to talk to the Romulan leaders, then Spock has time to travel from Romulas to Vulcan, then there's the time to outfit the ship, then there's the time it takes to travel back to the Empire.
How in the world do you figure the star that supernova-ed was the star Romulas orbited, only approximately 150 million kilometers (or whatever) from Romulas? The events in Spock's narrative would have taken many days, more likely weeks/months.
Even if the shockwave was moving well below light speed, Romulas would have been hit by the heat and other radiation from the supernova after just several minutes. After one revolution Romulas would have been dead.