The logic becomes sound once you take into account everything else I wrote before that sentence, and I sincerely hope that everyone reads posts for the purpose of understanding the gist of them, not for the sake of rebuking every line written.
The gender is, and I reiterate, a more or less unimportant piece of information around here, here being the forums dedicated to AirVPN products and everything related to them or the wider area of topics related to VPNs. Plus, as written, this piece of info is invisible to the whole community when set. When the purpose of that info is for the user to be referred to correctly, it doesn't help that user here if it's not seen by anyone. So the choice of whether I set it to "Non-binary" or "Not telling" doesn't matter in the context of these forums if the outcome – no one will see it, anyway – is the same.
If we talked about a context outside of the current environment, then yes, of course, having more choices is a valid request. But as the software does not provide that option, Not telling seems to be the closest thing to Non-binary. The other choices are binary, after all.