How can one respond to someone who claims that the Prophet, may God's prayers and peace be upon him, singled out some of his companions with certain knowledge to the exclusion of others?

No sane person can imagine that the Messenger, peace and blessings be upon him, concealed anything from the rulings of the Sharia or from the message of his Lord that they needed in their creed, worship, transactions, or morals. This is because such an act would contradict the covenant that Allah took from the bearers of His religion. As for what the Messenger, peace and blessings be upon him, specifically disclosed to some of his companions, such as Hudhayfah, Abu Hurairah, and Mu'adh, it was either something upon which no action is built, or it was derived from other texts. They were specifically informed of it explicitly for legitimate purposes and benefits.