They did, but I'm pretty convinced they were wrong (really, that the flight simulator they were using to verify it was, since it's not really meant for that). That plane has no roll stability; almost all of the lifting force is on the left, so it would probably spin clockwise around the fuselage till it falls around the sky.

Calculating aircraft stability along 6 axis (x,y, and z translation and rotation) is complicated, so hobby-level aircraft simulations often have some simplifying assumptions. In this case, I think it assumed that a symmetrical fixed-wing aircraft has roll stability, and people design symmetrical aircraft, so the simulation can just assume roll stability and not actually mathematically verify it (fair assumption, tbh). This aircraft violates those assumptions, so the simulation doesn't account for it. All models have assumptions, and all models are wrong, but some models are useful.

*I am not an expert in aircraft stability, so I might be wrong, but I do have a degree in aerospace engineering, have taken several courses in aircraft stability and control (the first of which I still have nightmares about), and have written aircraft and missile flight simulations.