Final answer:
The line from Siddhartha reflects the character's enlightened state of being attuned with the atman and the universe, indicating deep introspection and spiritual advancement.
Step-by-step explanation:
The line 'He already knew how to feel atman in the depths, it was being indestructible, one with the universe' describes Siddhartha's character as one who is deeply introspective and enlightened. Through the context provided by Vedic teachings, we can understand that Siddhartha has achieved a level of self-reflection and unity with the cosmos, indicating his spiritual advancement and his understanding of the non-duality between self and universe. This reflects his sage-like qualities, striving for harmony with all existence and suggesting a maturity in his worldview far beyond the common societal outlook.