As the MAX17263 reports 2.5 times the voltage applied to the CELLX pin and the voltage divider is connected to this pin, for 6S operation the 2.8MΩ on top of 200kΩ resistance is correct.
For example; assuming 4.2V per cell a 6S pack would have a max voltage of 25.2V and the voltage divider would give: 25.2 * (200kΩ / (200kΩ + 2.8MΩ)) = 1.68V at CELLX pin, so the MAX17263 would report 1.68 * 2.5 = 4.2V as intended.