Spock's line is meant to mean that there's 10,000 Vulcans (as opposed to Romulans) left altogether. Why else would he say he's a member of an endangered species? If China were destroyed would a surviving Chinese citizen claim to be a member of an endangered species if there were still 5 billion other humans left?
But we know for a fact that there are plenty of other Vulcans outside of the 40 Eridani system. The Romulans are
Vulcans: descended from Vulcan colonists, retaining many pre-Surakian cultural traditions, speaking languages closely similar to standard Vulcan, and interfertile with Vulcans (cf Saavik). Romulan culture has developed quite differently from the post-Surakian Vulcan norms, but that doesn't relate to Vulcan biology--there are still plenty of points the two civilizations have in common besides.
Who knows? Quite possibly, given the relative size of the Vulcan sphere of influence versus the RSE, there are many, many more Vulcans of Romulan background than there are Vulcans of more conservative 40 Eridanite stock. It might well be true that, centuries after the Vulcan Reformation, most Vulcans still
don't follow Surak's path. Dominant in one of the traditional superpowers, major players in another, and with all manner of related cultures Preserver-spread or not (Rigellians, Mintakans, et cetera), the Vulcans as a species are demonstrably not endangered. The Borg invasion was probably (possibly?) the first time since the successful colonization of Romulus that the Vulcan species' survival was threatened.
Even with the destruction of Vulcan proper, and assuming no substantial Vulcan settlements within or without the 40 Eridani system, the Vulcan species is doing just fine.
Old Spock knows this. Young Spock, presumably, knows this--the Narada
's early "contact" seems to have advanced Federation knowledge of Romulan background somewhat, the Vulcans also have their own inside sources and Spock being well-positioned in Vulcan society. Why, knowing all of these facts which demonstrate that despite the homeworld's loss the species remains, would Young Spock say that his species is endangered? There's no logic in that.