The assumptions about timing are the part I'm not sold on yet.

Assume that you do have a vigorous standard-dynamo just before impact that can re-magnetize the northern basin's surface. You then still have a highly damaged area of mantle material which could effuse volcanically longer than the dynamo survives. Not to mention that the sediment overburden of this magnetized layer has grown rather thick in the G-years since which would make possible a multiplicity of demagnetizing phenomena.

I'm sure it's a rock solid simulation and it IS a stimulating and clever idea but we're really going to need some ground truth before there is any certainty to apply to some of these theories.