Only creatures that are on the battlefield as Flaying Tendrils resolves will get -2/-2. However, any creature that would die that turn will be exiled, even if it wasn’t on the battlefield or wasn’t a creature as Flaying Tendrils resolved.

Creatures that Flaying Tendrils would cause to die will be exiled instead. The game doesn’t check to see which creatures die until after Flaying Tendrils has finished resolving.