These two notions are often lumped together (maybe rightly so..?), but are, in fact, quite distinct. One's prowess at reading does not impart knowledge of theory, harmony, musical styles (although helps considerably when studying those...), any more than the inverse; the study of music theory does not automatically make one a good reader, even less a good sight reader. Both skills, plus, of course, a degree of playing ability, are very useful indeed, but separate notions. Just sayin'.