In my opinion that's a better argument against the usefulness of a supreme court than it is a justification for allowing leeway in censorship.

The justice is claiming that said illegal content cannot be described or identified in law, that it must be up to a judge to make that call. Such a system is insane to those being ruled by it - we can't know if we are breaking the law, but at any time a judge could decide of their own accord that we are.

We must live under a system of laws that can be comprehensible enough for a reasonable person to be able to tell when they cross the line and are likely breaking the law.