If sburb destroyed all of the trolls (besides the 12 main ones) when it was first played, how did the condescension survive?

Sburb (actually Sgrub in this situation) didn’t kill all the trolls, it killed everything on Alternia. It was Feferi’s lusus that killed all the trolls. Before it died it released the Vast Glub which is a psychic wave that kills anybody lower than a Fuchsia Blood. (Condy of course being a Fuchsia Blood was immune to the attack)