Respectfully disagree. Teaching and explaining the current system to new and lower-level folks is arguably one of the most important skills for a senior developer. As for the system, they're not redesigning anything, they're teaching something that (as a senior) they should already know, as if we're juniors. Think typical box-arrow system architecture diagram, DB schema, that sort of thing.

Explaining the system is certainly something that happens, but in terms of hours/month I'd say it's something you do less of than leetcode-like coding work. Actually reconsidering your core design in response to some changing fundamentals, which sounded like it's what you're testing in this interview, is something that you do a few times a year at best.