When looking at diet fads or wellness trends, this statement is often used to gain credibility for seemingly outlandish ideas. I think this only a cautionary statement against close-minded and rigid thinking, a reminder that it is not wise and maybe even dangerous to stay wedded to a particular line of thought.
It is not uncommon to become attracted to one way of thinking either due to the seeming simplicity of the solution, charisma of thought leaders, desire to be part of a highly cohesive group or the sense of exclusivity it imparts. This is normal. What's not normal is the obsessive pursuit of evidence in support of that singular way of thinking and blatant disregard of any contradictory evidence.
Interestingly enough food really affects our mind and our behavior. Unbalanced eating may lead to rigid thinking and the eating disorders such as orthorexia, the unhealthy fixation on eating healthy. That reminds me of one more quote:
"This is deathless: the liberation of the mind through lack of clinging." ~ Buddha
So, go ahead: notice the ridiculous to gain knowledge, use opposition to sharpen your logic, and do not forget to question the self-evident. Truth arises when we strive to disprove it rather than through the myopic quest of looking for supporting evidence.
"All compounded things are subject to decay. Strive with diligence!" ~ Buddha
Gluten-Free vs. Freedom