In Siddhartha, spirituality is closely linked to wisdom. Those that have achieved enlightenment are also wise. Spirituality is also akin to the Holy Grail in Siddhartha because it’s very, very hard to attain. One cannot become spiritual by simply studying many books, but instead must have a personal revelation understood by the entire spirit (hence "spirituality").
While it is not the path Siddhartha chooses, it is possible for an ascetic to attain enlightenment.