It should already clear out Redis. If it doesn't, that's a bug.
Although to be clear, it "clears" the Redis cache by changing the prefix used for all Redis keys. So you will still have a lot of data in Redis just after you cleared caches, but the Redis TTL will clear them out eventually because Invision Community will no longer ready/write to those specific keys.
So if you spot lots of data still in Redis after hitting "Clear Caches" then that's not a bug.
If you clear caches and Redis is still returning 'old' data to Invision Community directly, that is a bug.