LDC1612: absolute positioning: use a differential 2 channel construction

I'm not exactly sure what the original author meant by that statement, and I don't know of any additional information on that topic.

A single channel can detect a change in linear, one-dimensional position.
I'm guessing what the author meant was that you would need a two-channel system to detect a change in two-dimensional  {x,y} position.

I'm emphasizing change in position because inductive sensors like the LDC1612 are better at that, compared to detecting a target's stationary position.

You can sometimes use LDCs to detect stationary position, but it is dependent on the application & requirements.
It also requires an up-front calibration. An example of this is the 1-Degree Dial Reference Design cited just above the sentence you mentioned.
Note that calibration may need to be repeated if system parameters drift over time.

Regards,
John