It is. But when you have one interface, that interface can be given the number 0. So you have a total of 1 interface, even though you've only counted up to 0.

Edit: I misunderstood. I'll leave the comment up. But I originally interpreted the story as meaning that each interface needed to be assigned an unsigned 16-bit id, which allows for a total of 65536. That was just inference on my part though. It literally says that more than 65535 are not allowed.