“That is from the stories of the unseen which We reveal to you [O Prophet]”
Cited (with Hud 11:49 and Al-Ankabut 29:48) to rebut a narrower objection -- that an unlettered 11-year-old could not have absorbed such detail -- rather than to vouch for the Buhayra account itself, which the lecture treats with real skepticism.
This is the Qur'an's own commentary at the close of Surah Yusuf, affirming the Prophet ﷺ could not have known this story except by revelation -- directly answering the test described at this event.