If we go to some JnAni to resolve the perplexity, he is not knowledgeable!
Or perhaps, he knows only to that little extent that the Almighty has opened out for him! Probably he (the JnAni) does not himself want to know anything more!
Nor does the seeker , who just received the information just because the JnAni condescended to tell him something, develop any further interest in it, to seek more knowledge! In this state of affairs, the vague knowledge itself becomes and remains the complete knowledge!
It is in that manner, when everybody was thinking that the mUrdha nADi that goes to the head was itself the sushhumnA of the yoga-shAstra, it was at that time that our Acharya manifested on Earth!
He was all-knowing even at birth. There was nothing which was not known to him. However, having manifested as a human being to show the way to humans, he had to show that he learnt everything only from the Guru.
First he studied several shAstras, as a Brahmachari, staying with a guru (*gurukulavAsaM*) and then from a sannyAsi-guru he took over the Brahma-vidyA. Thereafter he wrote the Bhashyas as per the orders of the Guru.