Where did I explicitly or implicitly suggest that the divider was significant source of noise, or for that matter even address it? My concern was always about how reducing the voltage range makes the analog signal more susceptible to externally generated noise (e.g. from EMI), not the amount of noise generated internally by the circuit.
But the divider might reduce the impedance by a factor of three - thus 3 times less susceptible to capacitive pickup...
EMI will be picked up externally from long cables and the environment principally - assuming you've got good ground-plane in the multiplexer circuit you've got control of the pickup there, so a divider isn't going to be the place noise enters the system, except of course if the thermal voltage noise becomes an issue (if you were amplifying microvolt AC signals rather than measuring 15V DC then I would be much more worried about noise).
Without know the input and output impedances and noise sources your original paragraph seemed a very specious argument for avoiding a divider circuit - certainly there's no automatic factor of three involved in precision.