n" ot pr'sei v"se mudrosti, jako pogl'shch'se, iznosim'ie ... , which, translated verbatim, means "We take all wisdom from the chest as though it were swallowed."
There is absolutely no real commentary on this episode.
I suppose this is an invective against the plot of the Solunskaja Legenda, which is the more probable that the whole part of VC related to the Khazarian mission is written on the substrate of the Byzantine antimonophysite hagiography (s. M. van Esbroeck, Le substrat hagiographique de la mission khazare..., Analecta bollandiana 1986).
the complete text of the episode, M. Kantor's tr. but corrected "absorbed" to "swallowed":
And furthermore the Khazar said: "Why is it you hold the Scriptures in hand, and recite all parables from it? However, we do not do so, but take all wisdom from the heart as though it were swallowed. We do not pride ourselves in writing as you do." And the Philosopher said to him: "I shall answer you in regard to this. If you meet a naked man and he says: 'I have many garments and gold,' would you believe him, seeing him naked?" He said: "No." Then Constantine said to him: "So I say unto you. If you have swallowed all wisdom as you boast, tell me how many generations are there from Adam to Moses, and how many years did each generation endure?" Unable to answer this, the Khazar fell silent.
PS The only, to my knowledge, explanation of this episode is provided by C. Zuckermann (in his REB 1995 article, p. 244) but it does not take into account the motive of swallowing.