The problem isn't demonstrating understanding of what a cache does conceptually, in fact any SWE worth their salt ought to be able to explain caching. The problem is if the candidate's decision to add a cache causes the system behavior to become obviously incorrect upon an ounce of further inspection, because this demonstrates a lack of foresight about local maxima vs global maxima, and systems are all about trade-offs at the macro scale.
Caching is an easy thing to blurt out in an interview setting, but not all problem spaces benefit from caching and caching often isn't the only available solution, or the most ideal one.
I honestly can't decide which would be worse: a developer who literally doesn't know what a cache is or one who installs caches with bad invalidation policies.
