I really have no idea how that RTC clock operates, but I can give you some info that might help you...
The I2C bus is designed in such a way that the I2C master (the RPi in this case), is what drives the voltage (brings it high, which would be 3V3 for the RPi). I2C slaves (the RTC clock here) "send" data by bringing the data line low (GND). So frying the Pi would not be a concern unless the I2C slave tries to draw some ungodly amount of current (which the Pi would be happy to deliver, at the expense of smoking itself in the process). This is because the RTC would never actually supply that deadly 5V that is sure to hurt your pi.
To sum it up, your RPi is delivering the voltage to the RTC, not the other way around.
As long as you power the RTC from the Pi's 3V3 rail, and the RTC doesn't do anything crazy with changing voltages, you should be safe.
Hope this helps :)