I haven't checked the context, but that indicates a flaw in either their framework or the way they draw this conclusion. We know for a provable fact that some problems (not necessarily interesting or useful ones) can be solved exponentially faster on a quantum computer than a classical computer: Deutsch-Jozsa algorithm (or it's generalization: Simon's algorithm) demonstrates this.
We know problems can be solved faster on a "quantum computer" as the term is defined; we don't know that the version of QM that our reality runs on actually allows us to create such a computer, or at least scale it up to a demonstrably (not provably! Demonstrably!) superclassical level. That's what the Quantum Supremacy debate is about.
