Can't speak to the validity of the explanation, but they claim there's a bug in their SDK that is keeping them from distributing the app in the newly required .aab format (as opposed to the .apk they had previously been updating).

Then there's also the possibility that they either don't feel like being beholden to an app store or there's some part of the app that wouldn't pass muster on the Play Store. No idea, really. I don't have any DJI products, but it would be interesting if the reason has something to do with app functionality (as opposed to just being too cheap/lazy to update the SDK or wanting to bypass the approval process).