As I’ve said before, while this isn’t bad advice as such, it is something that was plucked out of the air and has no scientific basis. It tends to be presented as something that is essential to healthy living rather than a nice-to-have ideal, and the consequences of not adhering to it are greatly exaggerated. Also, made-up “rules” of this kind are likely to undermine the credibility of all health messages, however sound their scientific backing. When you were a small child, if you were told not to do something, but given no better reason than “because mummy says so”, you would never have found it very convincing.
I have also reported in the past that a large-scale study found that adhering to this guideline only reduced cancer risk by 2.5%, which is statistically insignificant.