The relevant clauses read as follows:
b) You must cause any work that you distribute or publish, that in whole or
in part contains or is derived from the Program or any part thereof, to
be licensed as a whole at no charge to all third parties under the terms
of this License.
[...]
These requirements apply to the modified work as a whole. If identifiable
sections of that work are not derived from the Program, and can be
reasonably considered independent and separate works in themselves, then
this License, and its terms, do not apply to those sections when you
distribute them as separate works. But when you distribute the same
sections as part of a whole which is a work based on the Program, the
distribution of the whole must be on the terms of this License, whose
permissions for other licensees extend to the entire whole, and thus to
each and every part regardless of who wrote it.
The post on which we're commenting says that "derived" comes from copyright law, and not from the text of the GPL, because the GPL cannot supercede copyright law. What I'm pointing out is that's incorrect - the definition of "derived" comes from what the license itself considers derived. This is because the license puts conditions on "derived from" works in the above term. This is true even if the license says "see copyright law for definition of term X".
When trying to determine what "derived works" means, the intent of the license is relevant. The license cannot restrict your rights beyond what copyright law allows it to do, but it can grant you additional rights on terms of its own choosing, including very restrictive terms.
Importantly for this conversation, something that is derived from "any part" of the program may or may not be a "derived work" of the whole under US copyright law, but may be a "derived work" for the purposes of the above clauses regardless. Plus the latter paragraph has an AND addendum - "AND can be reasonably considered independent and separate works in themselves" - which can restrict beyond the referenced legal definition of "derived".