They are also deploying some rendering techs and they are working with an engine they are intimately familiar with. Theyve been testing for performance for some time and even in a very early build they could already support alot of people clustered together spamming spells with a "manageable" framerate.
Thats what ive found out since i asked that very same quests in this thread a little earlier. :P Still, ill believe it when i see it. But i do believe that ANet is doing everything they can to make it work, if anyone can do it they can. Afterall, they have alot riding on it aswell.
edit:
http://www.youtube.com/watch?v=fU2L7IdYDVc
They are using that in GW2.