I think that is correct. "Derivative work" is a legal term in copyright law that the framers of the GPL were wise not to try to redefine in the text of the lic

I think that is correct. "Derivative work" is a legal term in copyright law that the framers of the GPL were wise not to try to redefine in the text of the license. They seem to like the term "derived work", but do not define that either, probably for the same reason. According to the U.S. Copyright Office, copyright only subsists in derivative works as such to the extent they show some independent creativity, which suggests that a non-mere-aggregation that is a mechanical or near mechanical combination of works may not be a legally protected derivative work at all, i.e. may be the sort of work you can prepare without permission from the copyright holder under Section 106 or some other legal allowance. All assuming you own legitimate copies of or have a proper license for the component works, of course.