Mueller has the power to say something is not solid, but adheres to DOJ policy against declaring something to be a solid.

He looks at ice and says "no finding" because he knows its a solid but can't say so.

He looks at water and says "not a solid" because he knows water is a liquid.

He can't determine if steam is a vapor or suspended solid particles, so he makes no finding.

By MMM logic steam = ice.

Or alternatively...

He rules that water is not a solid, makes no finding on steam, and by MMM logic that automatically means that steam is solid.