Nice photo, and for what it's worth around here, I've seen the effect in person, in Kenya. It looked a lot like that, except the mountain was a bit closer and our cloud cover wasn't quite as tidy and uniform. On the other hand, we could watch it from earlier, before the sun got low enough to cast a shadow, then the cloud bank being lit from its edge with a shadowed volume running through it, finally the effect shown in the picture, where the clouds are no longer back or internally lit but lit from below with a shadow cast on their underside.
Presumably this is another case of bendy light bending in whatever way is necessary to make the flat earth appear round to anyone who doesn't know better.