"The correct way to say this would be 'what constitutes a derivative work under *copyright law*?' - the license in question is irrelevant."
Technically true, but in practice, what matters is both what copyright law says, and what the license says. Fundamentally, copyright law defines the "upper bound", in this case, what can or can't be restricted by law. But within this upper bound, a license can permit certain actions that the copyright law would allow to forbid. And as a practical measure, laws are often fuzzy; the exact boundary of "derivative" is sometimes unclear, and few people are willing to go to court over such interpretations... so if there is a lack of clarity, a license will often have a practical effect.
There are lots of proprietary libraries that have run-time fees, and their legal theory about derivative works is exactly the same as the GPL's view about linking to libraries. I don't know if a court case has ruled specifically on that point, but given the widespread practice, I believe a court is likely to accept the interpretation that an executable that links to a library is, in total, a derivative work of that library. After all, many people have been making that assumption for decades.
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