Well, red matter is a totally fictional creation. Maybe it has a totally fictional effect of being a giant vacuum cleaner and sucking everything back into itself. We see what it did to Vulcan, basically it vacuumed Vulcan into oblivion.
As I said, the movie explicitly showed that the "suction" only worked on things close to it. Vulcan was sucked in while the
Enterprise had no trouble escaping its orbit. The
Narada was sucked in while the
Enterprise was unaffected -- and then the
Enterprise inexplicably drew close to the black hole while manuevering away and only
then got caught in its gravity. So no, it doesn't suck in "everything," only nearby things. The movie shows us that unambiguously, twice. So the supernova's gamma and particle radiation, traveling at or near the speed of light, would be far out of range of the red matter's gravity pretty much immediately, long before Spock got there.
(Plus, as I said, it is impossible to "suck in" gamma radiation from behind, because light cannot slow down.)
Or maybe the mind meld was not a step by step retelling of the facts. Maybe the supernova happened quicker than Spock anticipated and he simply released the red matter too late to do any good.
Hmm... okay, you may actually be onto something there. We already know the mind meld's sequence of events is out of order, because it starts with the supernova and then shows Spock promising to help Romulus, whereas
Picard proved that there were years of advance warning and rescue plans in motion before the supernova happened. (Presumably the Vulcan Science Academy took over the effort once Starfleet reneged on the evacuation.)
So if the supernova happened, say, moments after Spock fired the red matter and before it reached the star, that could account for it. It would be a heck of a coincidence, but I guess no more so than the heroes
succeeding with only one second left on the countdown.
I have to admit I never liked red matter. The very name just sounds ridiculous. I know the Abrams team was a bit critical of the Berman teams over-reliance on technobabble. And yes, sometimes the technobabble was a bit over the top. But red matter was like the anti-technobabble. We'll call it red matter and that will explain everything. Um, perhaps a little more exposition would have been warranted in this case. It's usually not a good idea to force your audience to have to figure something out because you didn't provide enough information.
Abrams likes his "mystery boxes." He fills his work with Hitchcock-style MacGuffins, things that are important to the characters but unimportant to the audience except in how they motivate the characters, so the specifics of what they are or how they work are never explained. The most blatant example is the Rabbit's Foot in
Mission: Impossible III, where Simon Pegg gets a whole speech about how people have theories about what it might be but nobody's sure what it actually is.
Abrams is also fond of spherical red blobs of mystery liquid. The Rambaldi device in
Alias featured one too.