In many ways Siddhartha is a Buddha or an enlightened one with knowledge of truth and wisdom. Siddhartha like Gotama and Vasudeva has reached this stage of enlightenment and truth. He has gained the knowledge, of what he's been taught, and wisdom, a deeper understanding of what he's learned, to be at peace with his Self and to achieve Nirvana.
Just like the Buddha, Siddhartha had to go through life and achieve enlightenment alone. He had to obtain enlightenment through experiences and not teachings. He knew form his very first days with Gotama that he would not reach truth and Nirvana by listening to Gotama's teachings and instructions. He would have to experience all life had to offer lust, denial, happiness, and suffering on his own to realize his enlightenment to its full capacity. Siddhartha told Gotama,.
"I did not doubt you for one moment. Not for one moment did I doubt that you were the Buddha, that you have reached the highest goal, which so many thousands of Brahmins and Brahmins" sons are striving to reach. You have done so by your own seeking, in you own way, through thought, through meditation, through knowledge, through enlightenment. You have learned nothing through teachings, and so I think O Illustrious One, that nobody finds salvation through teachings. To nobody, O Illustrious One, can you communicate in words and teachings what happened to you in the hour of your enlightenment.".
Siddhartha knew that to reach his enlightenment he had to go out on his own and experience life in his own way. That enlightenment was not something that could be taught. It was a path that was different for everyone. He continues to tell Gotama, "The teachings of the enlightened Buddha embrace much, they teach much - how to live righteously, how to avoid evil. But there is one thing that this clear worthy instruction does not contain; it does not contain the secret of what the Illustrious One himself experienced- This is why Siddartha knew he couldn't stay with his friend Govinda as a disciple of Gotama.