ثُمَّ كَتَمَهُ includes ثُمَّ, which indicates sequence: a question was asked, the knowledge was possessed, and it was then intentionally withheld. This does not imply that every topic should be spoken about at every moment. The learner’s age, readiness, the question, and the setting all affect responsible teaching.
A complex issue may require careful explanation or referral to a specialist. But deliberately confusing students or withholding a necessary answer merely to preserve personal superiority is not sound teaching.
The lesson is to treat knowledge as a trust and convey it in a useful, responsible manner.