That´s not exactly what they stated, there was a post on this recently, that post has a link to the latest answer from fatshark in an older post. I´ll share that one here :
From what i can tell, the main issue is that it´d take to too much work for their studio to somehow get the mod reviewed, i suspect due to a lack of professional korean translators in the company, which is why they cant manage. And that i think would explain why it doesnt work. Because reviewing the mod is their policy and i suspect somehow demanded for them from possibly both steam and GW.