How to redirect electric field lines with a copper plane
An easier way to understand this is to realize that when a copper plate is placed above the circular conductors it's essentially creating a capacitor with capacitance between the three conductors. The middle conductor will be floating at DC, but touch sensors are AC measurements so DC doesn't really matter.  The diagram below shows three capacitors, but two of them are actually the same capacitance because of the  outer ring.

The nice this is this capacitance can be estimated easily (there is some cross capacitance that is not shown, but the effects will be minimal from this), all you need to know is the distance between the plate, the area of the copper and the relative permittivity of the materials (or air) in between.
\$ C = \epsilon_0\epsilon_r\frac{A}{d}\$
Since there is already a small amount of capacitance between the plates (Cm) adding a copper plate above will create more capacitance  and increase the current between the Tx and Rx plates. So a better way to look at it is instead of reflecting RF, you are increasing capacitance.
I haven't looked at the datasheet to know how the sensor data relates actual capacitance, I would guess that a higher sensor number indicates lower capacitance.
(Ctx and Crx could be modeled as a two capacitors in series which is essentially one capacitor so the values could be added together)