Here is proof they exist for the mod calling me a liar. As you can see I have two production buildings but the game thinks I can complete 22 8-hour productions in one 24 hour period.
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The Collector's Chest seems to be the one that wants the larger quantity lengthier productions, and most of the time if I do these at all, what I get goes right to the AD. If your city has only 2 production buildings then this is the kind of DC you probably should just skip. (I see it's just this last one before you collect the Challenger's Chest so I understand why it's driving you nuts.
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It's all in how you set things up.
For example, in my oldest city I do recurring quests with a CF while I'm collecting the city (I'm not a RQ grinder). I have pairs of supply producers arranged in a column set to 24-hr productions for the RQs. I could reset these to do challenges like the one above, but rather than lose the more valuable (to me) RQ production, I have a mix of blacksmiths and regular producers placed to do the chore -- this one I could do in 2 8-hr cycles.
They are there because events demand a lot of these production chores, and events seem to come one on the heels of the last. I can set these regular producers to 24-hr in between events for the RQs, and the blacksmiths are disposable so I can do the "build x (whatever) buildings" chores. Keeping the main bank of RQ producers set on 24-hr comes in handy for those 24-hr production event quest roadblocks they like to throw in, btw.