They've obviously retconned it. I think there was a writers strike during the crucial time ST:09 was made and so revisions to the script could not be made. It may be that they would have changed some of this back then. Ie, it is not a supernova that "threatens the Galaxy", and maybe changed it to make it clear that it was the star Romulus orbits that goes nova, so the red matter would still have left a lifeless Romulus.
Thus, stalling would make sense, since once it is used the Romulan star is destroyed along with all life on the planet. So the revised version of the plan was maybe to collapse it as late as possible to get as many off world as they could, but they waited too long, and it blew unexpectedly quickly toward the end.
Not sure. It still doesnt make alot of sense. They should have just said that someone fires a Trilithium warhead into Romulan star, only minutes exist to save everyone, and just forget the whole stupid red matter thing to begin with. Then some nutty Romulan tries to go back in time to change, Spock follows to stop him, etc, etc. IDK. It was a dumb script from the word go. Not easy to rescue it from it's own idiocy.
We thrashed thru alot of the stupidity of this thing in 09. Not worth getting to bent out of shape about it or the latest retcon. It's their F up, not ours.