Just because vampires can't be killed without special means doesn't mean that they can't just get hurt normally. Their bones can break, they can bleed and lose limbs, they can even be knocked outright unconscious. Spike and Angel have had plenty of trouble throughout the series just trying to keep ordinary opponents (ie demons and other vampires) from killing them. Against an army of these things, sure they'd last a while, and sure there's a chance they survived, but it's about as opposite as you can get from a good chance without being outright impossible. If we're to believe that Angel or Spike or even Illyria survived, given what we knew about the situation, we'd be believing it purely out of blind faith.
(Barring unforseen wild cards, of course...there is obviously a chance that the four people involved simply found a way to escape the battle, or the Powers for whatever reason decided to intervene, or even Buffy's people could have learned about the situation and decided to come to their rescue. Nothing's outright impossible.)
I think this will rule. Now I just need to find out what Loners is...
