English can be confusing. Here’s an example that might help -
In French, if you’re speaking to someone formally, you say “vous”, but if you’re speaking informally, you say “tu”.
Both translate as “you”, but “vous” is also used to address a group of people in French - so “vous” can refer to an individual, or a group, depending on context.
Some English speakers believe that the word “they” should only be used to refer to a group, rather than to a person whose gender is not being defined in the sentence. This is an overly rigid approach to language, because language is continuously evolving along with culture.
More importantly, this “English purism” approach is frequently used as an excuse to reject a transgender person’s preferred way of being seen. You’ll find the people who lean on this “argument” don’t typically care much about language purism in other contexts.
My two sats - hope it helps 🙏