I'm not really tracking where your confusion is. You can very much so do or "discover" something in practice that a theory describes without knowing that theory. I'd say it's even very common to "know" certain notes/chords/whatnot sound good with others and not learn "why" until later on, if ever.

My confusion is: why would you say that they don't know theory if they know the thing that the theory describes? If a person builds a radio by themselves, would you say that they don't know engineering because they didn't get a formal education on it? If a person figures out how to make a derivative, would you say they don't know mathematics because they didn't study it formally?