I suspect that some of them were just intimidated. Lamport is a well-known researcher in his field, and is giving a recorded presentation. Even a young postdoc

I suspect that some of them were just intimidated. Lamport is a well-known researcher in his field, and is giving a recorded presentation. Even a young postdoc might hesitate to suggest they fully understand something, only to have Lamport single them out and correct them on camera. In a different context, like a non-recorded seminar, you might see a more confident response.