The direction of some modern jazz improvisation requires some knowledge of music theory. It’s the same with certain ethnic music and modern classical. It all makes sense, it’s just a different musical language.
The last sentence is interesting, because it’s both correct and completely wrong. To rephrase your sentence, we could say that: “It all makes sense, provided you understand and appreciate that some, much or all of it is specifically designed to not have or make any sense.” - So, the result for the listener is either:
A. Just listen to it as a sound experience, without trying to make sense of it.
B. Try to make sense of it but expect to be frustrated.
C. Make some sort of sense of it but realise it will only be your personal sense, which will have little or nothing at all to do with the composer’s intended sense (or lack of it).
“A” is very difficult to achieve in practice. It appears we are preprogrammed to try and make sense of aural stimuli. “B” is the most common result, which is why it only appeals to a minority of people. “C” is probably the most common result amongst those who do like it and sometimes is the actual desired result. The composer might intend for you to make up your own meaning or “sense”.
From the early C20th, but with roots far earlier, the evolution of modern classical music was the breaking down and rejection of all the rules of the language of music. And, the same is broadly true of experimental (“Free”) Jazz from the late ‘50’s, which was partially/largely inspired by modern classical music. So, without any of the structure, “grammar” and ALL the other rules required by the definition of “a language” we can correctly argue that it’s not a language. Although, we could argue that it’s a language with whatever rules (or lack of them) any individual composer chooses. Either way, I’m not sure we can really say modern classical music (or Free Form Jazz) is “
a musical language”.
Ironically, as you mentioned him, one of the modern forefathers of this evolution of modern classical music was Charles Ives. Although, Ives’ works are not without meaning or sense, they certainly confuse, further break down and reject some/many of the rules of the “language of music” but they’re only the ancestor or first step of what was to come post World War II, with the likes of John Cage, Xenakis, Stockhausen, Boulez and many others.
G