I suspect a so-called expert's ability to solve problems within a domain (i.e., how "expert" they are at that domain) would be a function of their ability to manipulate terms in that domain which to me is akin to their ability to those terms clearly.
I can't imagine someone easily solving problems within a domain wherein the terms they use are giant spaghetti balls of complexity. Anything can always be broken down into further levels of abstractions (or prove me wrong on that).
I think the disagreement hinges on whether you believe the snippets from the GP post e.g. the "rods and cones" or caffeine examples are fundamentally valuable or not.
> There are fields where nobody can really do this. These are fields that nobody truly understands yet.
Curious what you would consider an example of such a field?
FWIW my anecdotal example of what I think the GP is complaining about... I read "The Elegant Universe": https://en.wikipedia.org/wiki/The_Elegant_Universe
It's a good book, and I enjoyed reading it. But over subsequent years I've had growing doubts over the true value of the "understanding" that it provided.
The frontiers of physics always strike me this way. The names and explanations are incredibly strained and bizarre. This is probably the canonical example and is perhaps why The Elegant Universe doesn't quite achieve what it sets out to achieve.
Another one from my own academic past is "complexity theory" where hand-wavey concepts like "emergency" or "higher order causation" get thrown around with a lot of confusion. We can clearly see there's some "there" here, but I don't think we really understand what it all means yet. Put stuff together, interesting higher-order stuff happens, and so now let's create a combinatorial explosion of jargon around it. Publish or perish.
Finally "consciousness studies" or anything else semi-fringe probably counts. Again there's certainly some "there" there, but we don't even know the right questions to ask. Something is happening to matter that makes it wonder what's happening to it.
Complexity is real, but understanding means being able to conceptualize a system at such a high level that it becomes possible to convey the most important key ideas simply. This may involve some amount of over-simplification (which should be indicated if present) and will undoubtedly leave out a tremendous amount of detail, but the conceptual essence should be there.
The inability to do this is IMHO a sign of either lack of true and deep understanding or communication ability.
There are fields where nobody can really do this. These are fields that nobody truly understands yet.
Edit:
Let me give you an example. I had a bad (but typical) math prof in my very first calculus class who covered the board with examples and took us through how to differentiate a function. I was completely confused and stuck. Then I called my father on the phone and asked him and he told me "a derivative is a rate of change." Then I got the rest of calculus.
The prof had talked about slope but that wasn't conceptual enough. I needed "rate of change." That is the essence of calculus. It's the most important concept. Calculus deals with rates of change and the inverse (integrals). Once this concept was communicated the rest of calculus became possible to understand.
If you can't do that for a given area, you don't understand it or (as was the case with this math prof) are a poor communicator.