Maybe I'm wrong, but I would not be surprised if forum moderation was a volunteer activity. Even core functionality like developing the lessons, for less popular languages, was done by the Duo community and given very little support from paid staff.
Yes, but moderating and screening for abusive content can't reliably left to unpaid volunteers if it needs to be good enough not to scare off parents and educators. I remember that this was the death knell to a Lego online game: People were making genitals quicker and more inventively than anybody could keep up.
In education technology, many seem to hope to get that from AI: the helpfulness of a forum of volunteers without the abuse. The rise of the "unsocial network".
They still have the forum style "help me out", but there are some where the responses are really a huge miss where I could see this AI helping out more by giving better answers.
They used to have an army of native-speaking volunteers to do this for free in the comments. I guess this is why they turned those off.