XB8 Modem

I’ve just been told that I cannot upgrade from the XB7 modem to the XB8 because I have 1000 mbps internet, and the XB8 is not compatible with this. I was told that in order to get the XB8 I would need to upgrade to 1200 mbps. Is this true? How is it possible that the XB8 isn’t compatible with gigabit internet?