Hi Konstantin,
PCF2127 operation voltage is indeed 1.8V(min) – 4.2V(max), which means the datasheet characteristics (meaning functionalities) cannot be guaranteed if 5V is supplied to the VDD pin, but it will not damage to the PCF2127.
The limiting value of +6.5V for VDD pin is for safety condition which means the VDD pin will have hard damage (permanently) if VDD pin is over +6.5V.
Best regards,
Tomas