Reading that story I had assumed it was due to the nature of the Warden Swords that can cut (and so presumably block) magic directly. But in terms of the RAW, it should be allowed in general, as Taran said, but as GM Id want some IC explanation for it. A vanilla mortal should not be able to deflect immaterial magic with a random trashcan lid; but by contrast any physical object should able to block physical evocation as the normal energy and material dictate (ie any metal object might block/ground a lightning attack; anything can block fire unless the fire overpowe4rs and burns/melts it).