This relates to – guess what – knowledge in Siddhartha. While knowledge can be conveyed using words, and such communication is necessary and useful, it is inherently deceptive. The wisest characters in Siddhartha tend to be extremely skeptical of words. (They use ESP instead. Just kidding.) Enlightenment lies in the realm of the un-communicable. It must be experienced.
Enlightenment in Siddhartha is communicated physically rather than verbally.