Imagine one has 10 request for thread {X} every second (probably a massive under estimation of the actual traffic). If you cache that single thread with a lifetime of a second you have instantly cut out 90% of your API usage for that thread.
Obviously the final benefit depends on what the actual distribution of {users} per {threads} per {time} -- but if your goal is to shave redundant API requests than it definitely makes sense, especially if the alternative is untenable in terms of cost.
Especially for an app like Reddit with millions of subreddits. There is no monolithic "reddit"; the experience is tailored to each user based on the subs they've joined. So your % of requests that will be asking for a cached resource is lower than other high-volume websites. I think your 99-to-1 estimate is _way_ off.