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[haruspex]

It's 75 degrees below horizontal, so why is it north instead of south?

It doesn't depend on the dip angle.
The direction of the magnetic field of a dipole is defined, by convention, to be out of its North pole into its South pole.
The North pole of a magnet is defined to be the end which is attracted to somewhere near Earth's geographic North.
But the North pole of one magnet is attracted to the South pole of another. It follows that regarding Earth as a magnet, its North pole is at its geographic South pole, and vice versa.
To put it another way, the North pole of a compass needle points in the local direction of the Earth's magnetic field, i.e., approximately geographic North.
(In the context of the question, the 75 degree dip means we're at a high northern latitude.)