Not so much that as Spock was shell-shocked and not making the most accurate logs. He didn't show very good judgement on any number of occasions after Vulcan's destruction: not putting Kirk in detention but sending him in a lifepod to the surface of a marginally habitable planet, trying to strangle Kirk on the latter's return on top of the conn console while the bridge crew and his father looked on aghast, making out with Uhura on the transporter pad, favouring killing Nero regardless of the possible good sense of keeping him alive.
Besides, Spock's statement is proved wrong by the simple fact billions of Romulans, as much members of he Vulcan species as any of the inhabitants of 40 Eridani, not only survived the attack on Vulcan but dominate a superpower. Vulcans are demonstrably not an endangered species.
Vulcan genetic diversity has been diminished. The Romulans were probably a genetically unrepresentative sample of the population of the planet Vulcan as much as they were ideologically unrepresentative (see founder's effect). Vulcan would house more genetically diverse lineages than the rest of Vulcan civilization; on Earth, there's far more diversity in Africa, home of the human species, than in the rest of the world combined. That's still different from saying Vulcans are endangered.