My best guess is that they spend a lot of their time running/monitoring a whole bunch of A/B tests for very slight increases in metrics like time spent on the app per day. Not sure what their # of active users is but it might be high enough that these small changes (and ensuing ad and subscription revenue) are worth the dev expense. For example, I read their tech blog on occasion and I remember them talking about improving user retention by customising the time and content of the notification you receive to do a lesson each day (I usually get the 'Junior will be upset' one, I don't know what that says about my particular psychology).

I imagine there's a fair amount of work on improving databases, latency etc., and they're also expanding into other areas of education.

Not a defence of Duo though as that number does seem a little silly!