This is only "true" if you want to pursue a classical repertoire. For anything else you don't need to know music theory to make music or read it for that matter. A good ear and instinctive knowledge is far more valuable. Just look outside of classical music, plenty of examples where the musician has no idea of the theoretical part, but has a great grasp of it practically.
"A good ear" and grasp of music theory go hand in hand. Strongly disagree that the latter is limited to classical music. The best musicians in jazz and pop music absolutely know how to incorporate the circle of fifths, types of cadences, Roman numeral harmony, etc., in their playing. That's... music theory! While there are musicians who can make it without that, they are the exception, not the rule.
Indeed, "theory" is not an all-or-nothing affair. There's a level of "theory" that's just learning why certain intervals are harmonious in the 12 tone system, and the names of things. I certainly learned those things, but if asked whether I know "theory," my answer is no. Virtually everything I know about harmony in jazz is due to hearing and recognizing recurring patterns.
In my case, I've gotten through 40 years of performing with jazz groups, so in some sense I'm doing OK, but I also know that I struggle with ultra-modern jazz harmonies. This came into pretty sharp relief when I played with some musicians who were composing all of their own tunes. I reach the end of my mental map, and then I have to fake it, or improvise directly from the melody.
But I agree that "ear" and perception of harmonic structure are closely related. It's hard to describe, and might make a psychologist cringe, but a musician develops a "mental ear." And I wouldn't recommend my approach to a young player. Most people want to become proficient in fewer than 40 years. ;-) There are things I can't do. I can't compose or arrange anything worth playing. Without exception, every musician I've played with who could compose or arrange decent jazz material has a music degree.
This is a bit like saying that all the best speakers know a lot of grammar. Maybe they do, but that’s not why they are able to put together complete sentences, let alone why they are able to move an audience with a speech.
I would add that music theory and sight-reading are orthogonal. My partner learned piano at a young age, and she can follow a sheet but won't know what chords she's playing. I learned guitar by ear but I'm always thinking about intervals/chords/modes. Obviously having both of these skills would be great.
Jazz was a genre mainly taught as an oral tradition, and of course improvisation was at the heart of it. Musicians played what they thought sounded good. That still didn’t stop George Russell’s book The Lydian Chromatic Concept of Tonal Organization from becoming hugely popular and influential among jazz musicians in the ’50s. Even when performers have an intuitive understanding of music, they can still benefit from explicit discussion of theory.
Also, a lot of jazz musicians wanted to eventually learn musical notation at least so that they could write their own lead sheets for copyright-claim purposes.
There are some great musicians who have no formal training, but far more who actually do have formal training.
Musical pedagogy follows classical music out of tradition, but there are plenty of contemporary pieces available as well.
The thing about the classical pieces is that they're good showcases and practice pieces for the fundamental techniques of music (hundreds of years of development will do that), which you absolutely will use in your musical career, regardless of whether you're even aware of it.
The difference is that when you can read and speak music, you can read, understand, and construct music far more easily than you could without the named concepts, nomenclature, and writing system. It's no different from the power that language and writing in general confers. An illiterate person can make himself understood, but a learned person can do so much more with far less effort.
> Musical pedagogy follows classical music out of tradition
i think at some levels, of pedagogy, there's also an element of "and because it is cheap to free".
my piano teacher bemoans how we only teach music from "dead white men" (a common refrain in some parts of the internet), but is hesitant to suggest i spend money to purchase anything, instead, referring me to IMSLP for everything.
if you want music from living folks, it is still under copyright, regardless of their color or gender. that costs more.
There are other reasons to learn written music, even if you're popular musician. For example, you may want to become a studio musician, so you need to learn to play quickly (in a few minutes) a complex piece of music to perform immediately. Studio musicians need to do this all the time, and reading from a score is the easiest way to achieve it. You may need to write music for other musicians (for example, wind instrument players and pianists). Finally, reading music will help you to learn theory and have a better understanding of music.
I think of this as analogous to self-supervised pretraining followed by training on a smaller labeled set. When you study theory you can ground it on music you've listened to throughout your life.
Also to improvise confidently you have to internalize the theory, not just understand it and memorize it at a conscious level.
I'd say "intuitive knowledge" is a good way to sum this up.
