I’m generally hesitant to get into this line of thinking (and others like it) because knowledge is such a thoroughly multi-dimensional space and usually the ends people are looking to move towards with these kinds of models aren’t terribly realistic.
I think the true answer is that it’s both hard to know what anyone knows about a given field and it also very rarely matters. It reminds me of the talk “Superintelligence: The Idea That Eats Smart People”—there’s a habit among intellectuals, academics, and learned professionals (usually in that order) to get so caught up in their work that they think it intrinsically matters, when, really, nothing does (at least not to everyone).
You can be very “knowledgeable” in a field, double down on the wrong side of a schism, and then see years of your work become nearly worthless when your mental framework is empirically proven wrong. That work might also turn out to be useful again decades later for secondary or even unrelated reasons; when and where are you more or less knowledgeable than your peers here?
And to circle back to the Superintelligence talk: we as humans are very adept at finding ways to survive and thrive despite all kinds of uncertainty and threats, and one of the best tools we have for that is ignoring things until they’re a major problem for us. In your radio intellectual example, I’d put forward that those kinds of situations arise because the presence or absence of such figureheads (or demagogues) doesn’t generally matter to most people most of the time. When such people become burdensome and overbearing in their demands, they are ousted—entire governments have been bloodily overthrown from within and without for such reasons. That feels inefficient to the person who thinks such fields and their heads matter, but it’s generally good enough.
My last point would just be that if it’s really hard to know how much more knowledgeable than you someone is, how can you have confidence that someone knows more about specific sub-niche X than you, and not just more about overall field Y? Einstein probably knew more about Physics on the whole than just about anyone outside of a group that could fit into a single lecture hall, but if he looked at a suspension bridge’s plans and wanted to “make corrections” I’d probably stick with a seasoned civil engineer unless they both agreed on review. The engineer would probably know more about the physics of suspension bridges in their home country than Einstein; if the latter was able to convince me otherwise that’s a question of societal status and political skill in general.
In response to your last point, I didn’t really get into differences between similar areas of knowledge in this post, it definitely becomes a messy topic. I’d definitely agree that for “making a suspension bridge”, I’d look at people who seem to have knowledge in “making suspension bridges” than knowledge in “physics, in general.”