but again, this turns into an argument of who could , can, can't or couldn't read.
With all due respect for the literate ( at various levels) music existed before its notation and there are many types of non western notations but music is performed by many players around the world who have no need to read or notate anything because the music is passed down from one musician to another and memorized in a oral or aural fashion.
All of this has been done since time immemorial in musical traditions which have a " canon".
Jazz is one of those tradition which uses an oral or aural tradition as well as a notated one.
Jazz players have several elements to use in their playing.
First there is a canon of musical pieces which we all more or less know and share, the so called standards. The standards came into use when this music was mostly dance music and people liked to recognise what they were dancing to.
Most of them were songs. You took a song and made it better! (Hey Jude?
) . If you knew the song you quickly got used to the changes and then using the form as a guideline the performer could easily perform a solo, exchange solos in various rhythmic forms.
Of course there were several ways to do this, most of this ways were rather intuitive and if not played in a " big band" setting, required no too much reading skills or no reading at all.
Most non readers are melodic-rythmic improvisers which is a very primitive way to simply take the basic melody and turn it around in many different ways. As long as you stick to changes and form you are ok.
Come Be-Bop or Big Band arranged music you need different set of skills.
There is much to say in favor of literacy ( and not only musical one) , but , in all forms of arts we have learned that there are many ways to skin a cat and the literate way is not, necessarily, a better way.