The voltage reference is present in the low-drop voltage regulator on the board, which is LD1117 (datasheet says that its temperature stability is typically around 0.5%, while it should be generally 1% accurate at 25 °C and 2% in all temperature range).
Regarding the protection - I would suggest a zener or suppressor diode between voltage divider and PoKeys, and a small choke would make it even better.
However, you should pay attention also to the power supply - suppressor and choke are mandatory here.
Generally, anything that goes in or out of PoKeys (except the network cable) must be protected by suppressor (or zener).