This is nice work. I would guess the only reason it isn't documented publicly is so that it can be changed at any time, without having to wait for existing users to migrate. However, given the sheer volume of people running outdated android app versions, I suspect it will remain stable for quite some time. Thanks for posting this.
What's also interesting here is that it revealed to me that as of early 2020, Facebook was still using the old (pre-2015) GraphQL syntax at least in part; because the examples you see in the linked fbchat code are of the old internal syntax before they opened it up to the world:
