Note I was using it as definition 2a in the linked article.
English has had several conventions when writing about someone of unknown gender in the singular. The old convention was to just use masculine pronouns. Then when people raised concerns about sexism, the awkward combination of using both male and female (i.e. 'he or she') came along. Since people are lazy and it's kind of awkward, the next step was to start using 'they' as a singular reference for unknown gender. Of course English has 'it', but that's also kind of awkward and impersonal for humans.
The concept of using 'they' as someone who doesn't identify as male or female (i.e. 'non-binary as mention in 2b) is the newest application of the concept - something that started even more recently with the rise in gender identity discussions.
This is by no means an accurate timeline, but I'd say the switch from 'he' to 'he or she' started in the later 60's to early 70's, and the switch to 'they' (as singular neuter alternative to 'it') in the early 90's.
Meanwhile, 'they' as non-binary gender term is the most recent, with all the gender identify stuff starting in the last 10-15 years, approximately.
That's grossly over-simplified, of course, and likely inaccurate, but it's roughly what I've seen in my lifetime. Singular 'they' apparently has a long history going back hundreds of years, at least
according to Wikipedia.
Of course my own usage was inconsistent in post #3, since I initially used the masculine pronoun. Since I'm casting doubts on the authenticity of the poster, I chose 'they' as a subtle suggestion that the poster may not even be male, let alone the actual (male) author.
