ENTP Logician
Advocate for Reason
As I already explained, I have lost touch with the original source, so cannot provide one. All I can tell you is that there was such a source originally. Having said that, the understanding of the quote in question comes as an intuitive one, and so stands upon its own. That process is an internal one, and amounts to one tapping into one's own Buddha nature. IOW, the Buddha nature by which he realized the message of the quote is the same nature everyone possesses as well. It is not knowledge that comes via Reason, Logic, or Analysis.
Siddhartha was not the only Buddha. There are many. There is Buddha, then there is universal Buddha nature.
Intuition is not a reliable means of confirming information.
Also if you do not have a source any skeptical person is justified in not believing you.