...at the time of the impact but that neither the main rotor nor the fenestron tail rotor were rotating.
....the helicopter struck the flat roof of the single story building with a high rate of
descent and low/negligible forward speed.
If the rotor is not rotating there is no gyroscopic stability. No/negligible (forward) velocity, the report says. Wind was a few knots. If still in the air, the a/c would topple. Now suppose that before impact the rotor was rotating very slowly. A slowly moving rotor blade would probably the first contact with a hard object which would bounce off, make a heck of a din, gather speed and the a/c could topple 'end over end' across hard objects making a tortured noise like a massive lawn scarifier over a stone path. I wonder if they need to widen the search a little for another point of first impact where a blade hit?