I would not call them the authority on the english language lol, but what are you talking about? The idea of being transgender was invented in the 1970's by John Money, there were no 1300 nonbinary people.
The relevant section of your definition is this:
I.2.c 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).
This was a recent addition. It did not exist in any older dictionary, or more moderate new dictionaries. I showed you this in an earlier comment, this revisionist history claiming they was always used in a singular fashion to avoid gendering people is insane, it literally couldn't have existed before the 70's because the concept hadn't been invented yet.
The word "they" being used in a singular manner is not something invented by transgender people. It has existed for a long time in natural conversation when talking about someone indeterminate. You can see it in examples like "Has anyone forgotten their coat?" or "Someone who's bought their own house would know." Activists couldn't hope to make people talk that way if they didn't already.
You can see it randomly used in old writing, too. For example, I just found this example of the word they being used in a singular fashion in Jane Eyre.
I observed when any one entered or left the apartment: I could even tell who they were; I could understand what was said when the speaker stood near to me; but I could not answer; to open my lips or move my limbs was equally impossible.
It was not uncontested, however; 19th century grammarians prescribed that people use "he" for indeterminate persons, and you will see this a lot in old writing, too.
Finally, I will say that transgender people have existed for longer than the 1970s. For trivial evidence, you can see how Christine Jorgensen got a sex change in 1954, making headlines as "Former G.I. Becomes Blonde Beauty" in a newspaper.
The relevant section of your definition is this:
I.2.c 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).
This was a recent addition. It did not exist in any older dictionary, or more moderate new dictionaries. I showed you this in an earlier comment, this revisionist history claiming they was always used in a singular fashion to avoid gendering people is insane, it literally couldn't have existed before the 70's because the concept hadn't been invented yet.