I am unsure as to what the question is or at least its pertinence to the dropping the basement window question. How about a picture from the exterior?

The angled drywall is merely a function of an adaption of the drywall to try to let more light in. If you are going to try to expand the exterior window opening, you are going to be looking at some cutting of block/concrete/masonry foundation wall. Not something for the DIY undertaking. The interior is probably a combination of insulation, framing, and sheetrock. Those are a bit more malleable and easy to work with.

The framed wall would be put on that angle and you will probably find it is easier to lay in some plywood to get the transition and angle spot on than just doing it with framing.