My hot take: there’s no such thing as “singular they” because you don’t need a special case for using plural pronouns with a single person; the basic usage already allows that. The plural pronouns refer to a group of people of any size. That includes a group of size 1.
A group of only one person is still a group of people.
That’s why it has always been correct to refer to a single person using the plural pronouns; you’re not directly referring to the person but rather to the group consisting of just that one person.
The reason this confuses people isn’t because the usage is incorrect but rather because what they were taught is incorrect.
People are taught that plural pronouns only refer to more than one person and that has always been wrong.
To see why that’s wrong, consider what happens when the size of the group is neither exactly one or more than one. For example if the group is actually empty or if you don’t know how many people are in it.
In both those cases you need to use the plural pronoun.
If the plural pronouns are a valid choice for both a group of size zero and a group of size two, then it would be ridiculous to argue that they are not a valid choice for a group of size one.