If we want to be super pedantic, gender is a spectrum because it has a range of different data points. The thing is, though, my argument is about what is taught to children about themselves. We know from census data (for example: [link to survey]) that people who do not identify as the gender linked to their biological sex is a tiny minority. I've said <95% because it's generous as hell and doesn't require quibbling over the precision of data sets. If we know for a fact that children will, over 95% of the time, have no trouble with their gender identity whatsoever, then it is outright confusing and wrong to tell them that their gender is on a spectrum. Little kids aren't going to grasp the differences between a bimodal distribution and a spectrum, especially when it seems most adults even have a problem with it. Trans, non-binary and intersex people, of course, do exist.