They/ them is just a way of referring to the third person. It is not exclusive to non binary people.
It's only used for an individual in the third person if they/their sex is not known - unless they have specifically requested to be referred to as they. Many, many people find it disrespectful, dismissive and even offensive to be referred to personally as 'they' when their sex is clear and known:
Mass letter: "Your child will be doing PE on Thursdays, so please make sure they bring their kit in with them" = standard use of 'they/them';
Individual letter: "Olivia has informed us of their asthma diagnosis, so we will ensure that their inhaler is kept safely in the office for when they need it" = poor/discriminatory use of language (assuming that Olivia does not identify as NB).
I don't buy the suggestion that they can't personalise, as the school send out regular positive texts to parents (primary school) saying e.g. "William did very well in their Spanish test today". How come they're able to personalise/auto-populate a text from any number of possible names, but not to link that with one of two/three pronouns?
The school/teacher could just as easily put he/she, but they obviously don't do this as they don't want to offend the small minority of NB children - but in so doing, happily offend the vast majority of children by referring to them with the standard NB pronoun.
I know I'll be called a crusty old dinosaur, but I feel the same about 'partners' being used as a catch-all to supposedly refer to all couples. Originally, forms would say 'spouse' or 'husband/wife', which did indeed unfairly exclude and 'other' those in relationships who weren't married; BUT instead of doing the obvious inclusive thing by saying 'spouse/partner', it was 'officially' decided that the majority and those choosing the 'traditional' route could simply give up their own clearly-expressed identities and have to make do with being referred to based upon somebody else's identity, which they're informed is supposedly 'neutral' and just shut up and go away if you disagree.