The evaluation board DC2832A states that C5 should be attached between GND.
What is the EMI when there is no capacitor between GND1 and GND2?
That discrete bridge capacitor between GND1 and GND2 provides a return path for common mode current and helps us attenuate the EMI at low frequencies. It's one of the ways to optimize that board…
You're right. Unfortunately, You cannot use that capacitor to get that Maximum Working Insulation Voltage of 1600 Vdc that the ltm2810 provides.
The package has a creepage of >16mm to get that…