I just read the Owners Manual regarding the DSC (page 73), says that:

"If the indicator lamp flashes:
ASC or DSC is regulating the drive torque and breaking forces."

So I take that to mean that the DSC is doing NO correction (when switched on) unless the indicator light is flashing.

So I guess I answered my own question above.

My point: unless the light was flashing when he drove it before, then switching it off should have made no difference. Of course, he was driving on different roads and conditions when he tested with the DSC off... and maybe that is the difference he experienced.