(non English speaking, here)

• I doubt many English speaking people had any idea back then that 'they' could be singular. Had they, I doubt that many 'singular' they proponents would have needed to fight so hard to make it accepted, you know.
• Maybe She she just decided they was not right for the way she wanted to write.

The references accompanying the quote from Wikipedia, at last the Oxford dictionary one make it rather clear it's a much more recent acceptation to use it as a 'personal singular':

2009–
Used with reference to a person whose sense of personal identity does not correspond to conventional sex and gender distinctions, and who has typically asked to be referred to as they (rather than as he or she). (...)
In the 21st century, other th– pronouns (and the possessive adjective their) are sometimes used to refer to a named individual, so as to avoid revealing or making an assumption about that person's gender;

Only mentioning a 'generic reference', aka mentioning an individual as a generic representative of some larger group (ie, a student) dating back from 1450...

Hmm, true but it was used in the case where the specific person being referred to was unknown. "Somebody left their umbrella." It was not used the way OP is talking about, for a gender neutral individual.