What the FSF wants the boundary of the GPL is irrelevant, since copyright law of the televant jurisdiction determines whether copyright license is even necessary, and the boundary of derivative works in copyright is both often unclear (as noted in sibling comments, different circuits in the US apply different tests that have not been harmonized by the Supreme Court) and differ between jurisdictions.

The GPL cannot create certainty that doesn't exist in the law on which the GPL relies to have any force.