Just to have a better understanding, is the point of series termination resistors to reduce interference from harmonics produced by fast rise and fall times more so than the frequency of the data signal?
Yes and no. You want "clean" signals at the sink side, so the receiver e.g. doesn't toggle a flip-flop twice on a single edge. To achieve that, one way is to reduce ringing and ground bouncing, this is what series resistors at the signal source are intended to do. And yes, doing so dampens higher frequency contents of the signals.
If you use a proper transmission line that is correctly terminated, one can use very high frequencies without causing interferences. Most PCB traces aren't well defined transmission lines, so one doesn't know the correct termination either. Modern CMOS stuff has quite fast edges, often in the ball park of 1ns fall/rise time. On a typical PCB a 1ns edge propagates by some fraction of speed of light, maybe 15 ... 20cm. Mistermination causes quite visible signal distortion then (if you have fast enough an oscilloscope). As long as your rise/fall time is slow enough in comparison to the trace length (shorter than 1/4 of the wave length of the highest frequency component), this effect can be mostly neglected. So if you can't make your trace shorter, make the edges slower.
Fast edges driven by an CMOS output cause current flow at approximate same frequencies through the VCC / GND pins of the device. Parasitic (package, bond wire, PCB trace) inductances cause voltage drop according to this, causing the IC internal potentials to deviate from the external GND / VCC. This is called "ground bouncing" and can cause interference on other signals. You want a solid GND plane, good VCC decoupling and very short traces from the plane and decoupling capacitors to the GND / VCC pins of the IC.
Series termination at the signal source roughly does a mix of both, slowing down the edges and approximately back terminating the signal trace. So the series resistor (or using less stronger driver settings) help with signal distortion through mismatched signal traces and ground bouncing.