It's an absolutely truism that we don't know what we don't know, but i also think that some don't know what the phrase properly means.
The problem is partly the Dunning-Kruger effect -- for various reasons, the "incompetent" tend to overestimate their competence. A few of the reasons might be that the person is simply stupid and they CAN'T register their degree of mental inferiority, or their psychology is such that clues of their ignorance fall on defensively-deaf ears, or they're so insulated they don't know what the range of possibility is, and self-esteem (a branch of the biological imperative to survive) forbids them to place themselves in a non-esteemable category.
WAAAAY back in the '80s, i read the book "Dress for Success" because ... well, i knew someone who didn't. One of the points made, which has stuck with me all this time, was that when looking down the social scale we see very clearly, but when looking UP it, we're definitely myopic. Seems to me this happens a lot in the diet-and-fitness world, when some young "undamaged" person or pushy musclehead knows what EVERYONE needs to be strong and healthy.
I've found it extremely illuminating that even people with the same gender, age, and symptoms don't react similarly to certain foods/supplements/pharmaceuticals/exercises -- hell, i find that i sometimes don't react as i did LAST WEEK, let alone ten years ago. So even though i'm a believer in the idea that one should start looking for answers to one's problems amongst the experiences of one's true peers, it really is ONLY a starting point.
No matter how much we read, no matter what our own experiences have been, we DON'T know what is going to be the magic bullet for somebody else. For Joe Bro to claim that he knows the answers because he's known an example of the demographic who succeeded on his plan only shows how out-of-touch he is. We can all see DOWN the river to our pasts in the valley below this hill we're climbing, but looking UP the river, our view is obscured by the mists coming off the rough water ahead. Somebody else's path, around the shoulder of the hill, may not be visible at all.