"He noted that the magnetic field does shift every half-million years or so, but "there's no evidence it's going to happen in December, and even if it were to be shifting, it takes thousands of years to do so. "

We know this from lava flow. It magnetically orients itself when it cools.
The mid-Atlantic ridge is a prime example of the flipping.

I am surprised that it takes "thousands of years".

The sun flips its poles every 11 years and the process is much faster.

..anyway..I suppose the evidence is in the rocks.