It seems strange to disregard an entire article written by a Professor of Psychology & Neuroscience because a quick google session found a link suggesting a contrary view to a specific point.
Indeed; one could say that he himself did not do his homework by reading the entirety of a text.
It's quite likely he meant 'overripe', which might be considered one form of 'unripeness' (though 'unripe' usually means pre-ripe).
Even though there are many sources touting unripe bananas as a folk cure for diarrhea, an article from the 'World Chiropractic Alliance' wouldn't be my go-to source.
This made me curious, so I googled it. The very first result is a scientific study demonstrating that unripe bananas are a great remedy for diarrhea: http://www.worldchiropracticalliance.org/tcj/2000/aug/aug200...
I stopped reading the article after this, fearing that the author did not do his homework.