Use-mention distinction
This is an example of the use-mention distinction. In the sentence "Who is they [sic].", "they" isn't being used as a pronoun, but instead is being used a word. That is, it's referring back to the use of the word "they" in the previous sentence ("Now they've decided to take on Ava Hessington personally.")
Notice in that sentence by A the "they" takes the plural form of the verb ("they are"/"they're"). This is always the case with "they" when it's used as a pronoun; it's always gramatically plural, even when it's used to refer to a single person. (Which, despite moaning by some pedants, is a perfectly valid English construct.)
In the sentence by B ("Who is they?"), the "they" isn't being used as a pronoun, but is being used to refer to the word "they" that was used by A in the previous sentence. The convention in English is that when you have this sort of "mention" case - where you use a word as a word as opposed to using it for its meaning - you should place that word in quotes or italics. So the closed captioner should really have rendered it as
Who is "they"?
or
Who is they?
The fact that they didn't is either due to limitations of the closed captioning system (for example an inability of the system they were using to represent quotes or italics), or simply because the captioner wasn't aware it was needed.
Note that the quotes/italics here aren't being used as "scare quotes" or as some sort of marking of incorrect usage, but as a literal quote of the word itself. As such, the speaker wouldn't actually make an air quote gesture, but probably would subtly emphasize the "they", either by saying it in a slightly different tone or pausing slightly before saying it.
To start, the sentences you gave are not how singular they is normally used. The verb simply takes the normal form for they. The sentences would be:
- They can write what they want.
- When they need help they get it.
As for
Does this come from having any strings leading into deep history of English where this existed?
It is actually explained in the article you linked to:
[W]e tend to miss that English speakers have been using they in the singular since English was anything we'd recognize as English. Back in Middle English, the Sir Amadace tale includes, “Each man in their degree.” The Bard has Antipholus of Syracuse in Comedy of Errors chirp, “There's not a man I meet but doth salute me / As I were their well-acquainted friend.” Thackeray has Rosalind toss off in Vanity Fair, “A person can't help their birth.” Whence the idea that all of these people were butchering the language?
So, yes, this use of they is quite old in English. The article goes on to explain why people started to think it was "wrong":
It was the schoolteacher and writer Anne Fisher whose English primer of 1745 began the notion that it's somehow bad to use they in the plural and that he stands for both men and women. Grammarians of Fisher's day tended to believe that real languages should pattern themselves after Latin and Ancient Greek, in which the words for they happened not to have experienced such developments.
Like so many nonsense-rules in English, that sadly have been taught to generations of students, the whole notion came from the misconception that English should be some form of Latin or Greek. "Never end a sentence with a preposition" is another example of those grammatical fancies that were drilled into the heads of unsuspecting students without any good reason.
Best Answer
You actually have the answer to your question, within your question.
Let me split the sentence : You can't cast aspersions/ on someone/ just because /they are/ wearing a cape.
Note: I have not split this sentence according to any grammatical convention, I just split it up for the purpose of the discussion.
In the sentence you are talking about "someone" and well "someone " is singular, so your sentence is talking about a general, unknown him/her.
Since we are talking about a singular him/her, the usage "a cape" is correct.
Now, how does "they are" which implies a group of people fit into this sentence? Well let me introduce you to 'Singular they '. It is somewhat similar to the usage of 'you'
eg: you are strong - Singular reference
you guys stink - referring to a group of people but it is the same 'you'
when we were kids, we used to make fun of our teacher behind their back and giggle. Every time this happens, the teacher would shout " If anybody thinks they are so smart, you teach the class I will listen!" (huh, fun times).
If you want to become a "Singular They Expert" you gotta check this out.
There is another more complicated concept to this "Singular They" called as epicene but explore at your own risk. I dont want do discuss further about epicene as I am not very knowledgeable with the grammatical nuances of an epicene.
Edit: I agree with @sgryzko 's comment, maybe the example I originally gave needs a little modification. The example suggested by @mplungjan is definitely an improvement - "When we were kids, we used to make fun of each and every teacher behind their back and giggle".