I looked, too. Sure no signs of the material being washed out of the sides of the gash. Sure looks like a tensile failure to me. What forces pulled that hill apart?
Gotta be a wild Saturday night party in the underworld.
From what the engineer said, you’ve pretty much got it. There are apparently springs where the soil meets the bedrock so the landslide is triggered on the surface of the bedrock. So your implication that there is a crack in the ground moving downhill rather than erosion from the suface seems correct from what the photo shows.