"Defense in depth" sounds, to me, like a phrase to justify multiple layers of imperfect security.

A single layer of cloth might not hold water, adding more layers of cloth may hold water for longer, but it's probably more cost effective to start with the right material.

> "Defense in depth" sounds, to me, like a phrase to justify multiple layers of imperfect security.

That’s absolutely correct! But you seem to be missing the fact that _all_ layers of security are always imperfect.