RC filter on ADC voltage reference pins: overkill or necessary?

That RC is a good idea. It's worth having, especially if you will be operating in an environment with EMI.

I suspect (although the datasheet doesn't say that explicitly) that the differential REF pins are connected (through a mux) to an InAmp. One problem with InAmps is that their common-mode rejection at higher frequencies is poor. InAmp can also rectify high frequency noise at its inputs, and that manifests itself as a DC offset. To deal with that, low-pass filters are added in front of InAmp inputs to cut the high frequency components.

This is similar to what you see in fig. 5-24 of the A Designer's Guide to Instrumentation Amplifiers.