If I understood correctly, what this new finding is supposed to explain is this:

The Borealis impact, if real, happened very early in Mars history. Early enough that the dynamo should have still been active. And it would have created a partial magma ocean, which certainly would have been magnetized by any field.

In contrast, Hellas and Argyre were later impacts, and so by then the field might have been gone.

On shakier memory for me, do impacts of that size wipe out fields from shock but don't cause enough melting for wide-scale remagnetization?

So, if this new finding holds, it explains how you can have an early huge impact yet not have magnetized northern plains.