Also why would I voluntarily talk about codebases authored by anyone other than me while having in-depth knowledge about it? Nobody looks under the hood of things that interests them, let alone be critical of it’s design.

Talking about product and system design is something that is vastly different than stand-alone codebase. If you as an interviewer are interested about something you have the capacity of directing the conversation flow toward that. Ask them introductory questions about a product/system, gauge their interest in it then ask their opinion about it.