"Horbus" is a useful placeholder name for the unnamed star in question, hence why I use it.
But the point is, the movie never said it
wasn't the Romulan sun. So there is no actual contradiction between the movie version and the
Picard version. It's merely a clarification. Contradicting what tie-ins have claimed or what people have assumed is not a continuity error. All that matters is the exact letter of the text. The Romulan sun is "a star" as much as any other star is. Lack of specificity is not a contradiction.
If Horbus was indeed the Romulan sun, then Spock could not save Romulus, since removing the sun would destroy the world anyways (albeit by making it uninhabitable, not by vaporizing it).
The planet as a physical entity is irrelevant. It's just a hunk of rock. What matters is the people, the civilization. "Save their planet" could mean "allow the people to survive long enough to evacuate."
Nero is very blunt that he blames Spock for both his family's death and for his homeworld's destruction, which makes no sense if the world was toast no matter what.
Nero was not a rational actor. He was crazed with grief and looking for someone to blame. Scapegoating is rarely sensible.
Although if anything,
Picard makes Nero's resentment of the Federation more reasonable. It's not very rational to blame Spock or the Federation for being unable to predict the exact moment of the supernova; nobody could do that. But it's entirely rational to blame the Federation for abandoning the evacuation effort after the synth attack and thereby leaving more Romulans to die when the supernova came.
PIC's casting Horbus as the Romulan sun does not fit with Star Trek (2009)'s backstory.
Of course it doesn't, but that's irrelevant. There are lots of things in Trek that don't fit with a precisely literal reading of earlier installments. "Mudd's Women" had Harry Mudd recognize Spock as "half-Vulcanian" on sight, implying that full Vulcans looked less human than Spock did. That was contradicted when we finally met other Vulcans. Data originally said he was "class of '78" at the Academy, then we learned the current calendar year was 2364. Trill hosts were portrayed in TNG as contributing nothing to the joined personality, but DS9 retconned it so that both personalities contributed (as well as abandoning the Trill's inability to be transported safely and redesigning their makeup). And so on. It's the prerogative of a work of fiction to tweak, refine, and improve its ideas.
Especially if those ideas are bad and stupid, like "The Alternative Factor" saying that a matter-antimatter reaction would destroy the universe, or ST V saying you could get to the center of the galaxy in 20 minutes, or "Threshold" saying that transwarp would turn people into salamanders. All those things were blatantly, repeatedly contradicted by later Trek because
they were stupid and didn't deserve to be acknowledged. And the same goes for the 2009 movie's sloppy, ludicrous, nonsensical account of the Romulan supernova. The version in
Picard is a thousand times better. Quality is a higher priority than continuity. Fiction is an imperfect human creation, and its creators have the same right as any other humans to fix their past mistakes, or to try to undo the damage caused by their predecessors' mistakes.
(There's also the question why no one in the PIC prequel novel seems worried about Horbus destroying the Galaxy,
Because that bit was also incredibly stupid and it
deserved to be ignored.
Anyway, I figure all the nonsense in the movie's mind meld scene can be handwaved because it
was just a mind meld. We were seeing Kirk's interpretation of Spock's thoughts through the meld. A brain is not a tape recorder storing information precisely; everything is filtered and interpreted and stored through a web of associations and symbols. Filtering one mind's thoughts through a second mind's perceptions would add a second layer of filtering and symbols, so what came through would be just general impressions, feelings, idioms, metaphors. An experienced melder like Spock could probably filter that out and distinguish the solid information from the chaff, but Kirk's mind was more inexperienced, so his interpretation of Spock's thoughts was probably quite flawed. Maybe he heard "a star" because he missed the bit specifying it was Romulus's star. Maybe he heard "threatened to destroy the Galaxy" because he caught a conceptual whiff of widespread danger and his mind read too much into it.