The theory is that it doesn't have a cognitive realm manifestation. Currently there is no evidence of that. I am not 100% sure where it mentions that shardblades cant cut aluminum, I think there was a WoB somewhere about that, but they can't because they cut using investiture, which means they can't cut aluminum any better than a normal sword can. Of course, living shards can change shape, so in theory, with a living shard you could just make it much sharper so it could cut aluminum. The point is though that shardblades can't cut it using magic like everything else.