[OkayPhysicist]

This really shouldn't be some surprise. You probably do it on a regular basis without noticing. Shakespeare uses it in Hamlet:

There's not a man I meet but doth salute me As if I were their well-acquainted friend

English has always used the singular they, especially (but not always) when the gender of the target of the pronoun is unknown.

[tempaccountabcd]

ƿrong. "they" is norþmannisċ. It sċuld be he, forðat hē and hēo ƿuld'fe melded into he, and forðat Englisċ is an Indo-Europisċ tung, ƿere ƿerelie is ðe first kin.