Thanks for linking to a quote. Here's what that article shows as using singular they:
"There's not a man I meet but doth salute me / As if I were their well-acquainted friend"
That's still not referring to a single, known person. The gender is known in this case due to "man", but it's still an unknown/unspecified man or men. I don't have a strong opinion about the recent push towards using singular they in this way, but I do really dislike the arguments for it that falsely try to add historical significance to their claims by invoking Chaucer or Shakespeare.