I've always seen this saying as being more about vague and general aspects of life as compared to professions and subjects where competency can actually be meas

I've always seen this saying as being more about vague and general aspects of life as compared to professions and subjects where competency can actually be measured in any real way. Like when it comes to living their life as an independent adult, no one would really know what they're doing, since everyone's situations are different enough and the markers of 'success' are vague enough that there is no one answer. Or maybe politics/running a country, where again, the only real goal anyone has is 'make the world better in some way' and everyone heavily disagrees on what 'better' looks like or how it can be achieved. On the other hand, I'd guess a pro athlete would have some idea what they're doing when it comes to their sport of choice, and a doctor would know at least something about medicine and how to treat illnesses. I don't think the saying is usually used much in these contexts.