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I think they are referring to a moving point on earth's surface where all 3 norths appear to be in the same direction. I agree it wasn't clear.

What's also unclear to me is how all 3 could reliably be colinear, but maybe it's an aspect of spherical geometry that eludes me.

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The locus of points where magnetic north aligns with true north is a (one dimensional) curve. On one side of the curve magnetic north is too far to the east, on the other side too far to the west.

Same for the locus of points where grid north matches true north.

The two curves meet in some finite number of isolated points, at each of these all three directions are the same.

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