The point is that potatoes are one of the top 4 food sources worldwide, and they collectively dwarf the percentages of the long tail of other food sources.

Proportions vary significantly continent-to-continent and culture-to-culture, such that it'd be more meaningless to try and put a more precise (but less accurate) number on it.

This is not difficult to parse out of the sentence if you choose to take a charitable rather than a pedantic perspective, which I recommend to you generally.