Is a Hafiz of the Quran concealing his memorization from people considered concealing knowledge, or is it correct to do so to avoid ostentation, especially if his work is far from religious scholarship?

The best approach for a hafiz (one who has memorized the Quran) is to make his memorization apparent if he is to be emulated or if he is to teach others, provided he is safe from riya' (ostentation). Concealment in acts of worship that are not obligatory to be displayed is better, unless there is certain intention of teaching and being emulated. In that case, displaying it is better because it is a transitive act of worship that awakens the heedless. This concealment is not considered the concealment of knowledge for which a warning has been issued, as the warning for concealing knowledge pertains specifically to knowledge that must be taught, such as the fundamentals of Islam and the rulings of prayer.