how can you say thats not the case when you said for youself that this guy plays the guitar yet doesn't know how to read music? Myself in music class the hardest part for me was learning how to read the music and when I couldn't I gave up. I'm a quitter yup.
Well, think of it this way. I can read music perfectly, in treble, bass, and alto clefs. I can read several notes at the same time. It is EASY for me. However, here is someone that I can't even begin to imitate. I can read music, which you say is difficult. He can play really well, which I say is difficult.
Intonation is always harder than reading music, especially on the higher wind instruments. It is even harder on bowed string instruments. You mentioned you played flute. Ok, reading high treble clef notes. That would be easy for me. But I'll be damned if I can keep the thing in tune. Roll it in, roll it out. Heck, it changes depending on which note you play. Most people can't even hear it at first. Building that ear is, in my opinion, the hardest thing.
I guess in the end, this is all my opinion. Maybe notereading is the hardest thing for the beginner. When you get to higher levels, everything changes. The rudiments are easy, such as notereading, counting rhythm, keeping time. The challenge comes with learning where the note is in the whole piece and pulling it up or pushing it down based on that. The challenge comes with learning how to blend, not only with your section, but with your entire group. The challenge comes with dynamics and expression, using these to not just create a string of notes, but a mood.
Notereading is a given. Playing the music "correctly" (right notes, right rhythm) doesn't make it music. Giving it a feeling does that.