In the classic sense, your elite geology fraternity has interpreted a DC waveform with ripple as an AC waveform.
Nope.
You have interpreted a trace left by an AC waveform with a ripple on a different DC waveform merely because the
trace of the AC waveform is lower in magnitude than the
actulaDC waveform. They are not comparable.
The trace of the AC waveform (that is, the magnitude of the frozen-in magnetic field in the rocks) cannot be meaningfully compared to the magnitude of the DC signal (the Earth's magnetic field today). We could, of course, calcute from the trace what the magnitude of the Earth's magnetic field was in the past, and indeed we have done so. For example,
How Are Geomagnetic Reversals Related to Field Intensity?. And we find that the Earth's magnetic field has indeed reversed, and was sometimes greater than the current strength and sometimes less.
The fact that the magnetic field of the rocks (in which the historic reversals are frozen) is less than the magnetic field of the Earth is irrelevant. Your "DC signal" is in fact an irregular and slow (in terms of human lifetimes) AC signal.