Based on my understanding of the plugin code it is the default. The way cache.Indexer works is it's continuously streaming resources from APIServer using Watch API and updates internal map. I think if Watch API is down it just sits there and doesn't purge anything but I haven't tested that. The 20 min expiry is probably referring to CodeDNS cache stanza which is a separate plugin[0].
[0] - https://coredns.io/plugins/cache