You have a good point; it is a weakness of this site's rules that we acknowledge and accept as necessary.
In an ideal world, we would have a nice strict rule. Something like:
At least, say, 500 people have to accept a claim as likely true before we think it is worth mentioning the claim and researching the evidence behind it. The OP has to prove that at least that number of people "believe" it.
Of course, that is almost impossible to do. We can only guess at how many people believe a claim. So we have to rely on proxies.
If it is on Twitter and has thousands of positive re-tweets, that seems like a reasonable proxy to suggest many people believe it.
But sometimes, we don't even have evidence that strong, so we accept even poorer evidence of belief.
If it is printed in a major newspaper, so tens of thousands of people read it... well, even if they don't believe it, it is worth having a place where they can check whether they should.
If it is stated by a politician or celebrity or someone who similarly has many people listening to their words, we accept that as a proxy measure that many people might believe it.
(Note that we want the claim to be notable, but we are using the less valuable a proxy of the person who said it being notable.)
So, your point is very valid: It might be that all of the readers of Business Insider rolled their eyes when reading about the diet and none of them believed it. No evidence has been provided to dispute that.
But to make things tractable for the question askers, we accept spoutings from a celebrity as good enough evidence of notability.