No. Reread what is written. The claim was that up to 8000 Hz is needed. Above that provides further clarity. That "further clarity" isn't needed to hear regular speech if you have otherwise normal hearing up to 8000 Hz but any increase in clarity can make a big difference when you have hearing deficiencies.
Case in point, the 4 who saw an increase in word scores had moderate to severe hearing loss on the standard (up to 8000 Hz) audiogram. For them any boost in clarity would make a big difference in trying to discern speech.
While the bulk of speech info is under 8000 Hz, there is definitely additional info at the higher frequencies. I posted one article from PubMed for you and would be happy to post others. Just let me know.
https://journals.sagepub.com/doi/abs/10.1177/019459987808600125