Seems ignorant and naive on the Romulan's part to not at least agree to try to corroborate his findings. What would that hurt?
I quote the Romulan senator from the comic
"
But I assure you that the best scientific minds on Romulus have studied the Hobus star and have not come to the same cataclysmic conclusions.
And decalithium is a rare and valuable raw material in the empire. We will not willingly hand it over to the Vulcans, despite your well-intentioned supplications"
And after Nero's intervention
" But the ambassador's conclusions are hasty, drastic
and require further investigation"
So it's not like they have not investigated the supernova. They have the assurances of the best Romulan scientists that's it's all good.
And they are after all politicians.
No matter what progress has been made in relations thanks to Spock, I think it's still too much to ask to ignore all Romulan scientists and believe a Vulcan and hand to him rare and valuable resources to take to his home planet.