He thinks conciseness is a physical property. It's not clear to me why he thinks only brains, or maybe only living things, can have this property.
He makes the analogy that a weather simulation in a computer can't make anything wet so therefore a computer program can't have a thought. My take is that when we think about rain we don't get wet either. Human minds are the same sort of thing as the weather simulation, not the same sort of thing as the weather. For me thoughts are simulations, or models, or operations on models, and that those activities are tractable to computation.
