So while I don’t disagree with your point that third-gendering can be invalidating to some binary trans people (and the author was being lazy / over-simplifying / ignorant and thus could have done better), I think it’s a little mistaken to focus so much on this small mistake and to direct that anger towards the pro-trans author (your ally) when the larger context is what matters and is still accurate - the dominant, oppressive gender concept is anti-trans, and the author is right to call out the anti-trans policy as anti-trans.
You are splitting hairs on the gender binary
I agree with both the above. Sure I am nit picking, but for good reasons (I explain below). But you are mistaken in assuming I am "angry" at the author. I am just expressing the only noteworthy thought I had about this article. I upvoted the thing!
you are using “trans” in a way that might be a bit more narrow than I was meaning
Well, normally I don't, in fact I recently explained that since biological sex is not a fixed binary it is absurd to assume that gender identity is.
umbrella term that encompasses gender non-conforming people, non-binary people, cross-dressers, drag performers, as well as people who transition socially and/or medically
This is a very well put together and comprehensive list, and I don't even think these terms are mutually exclusive. But I do make some conceptual distinction between (just an example) drag queens and trans women, I think it is more accurate to define "trans" in terms of gender identity not expression or performance. I would use "trans*" or "GNC" as an umbrella term, like in a future red book or style guide.
Since we can now use some shared terms, let me rephrase. 3rd-gendering is not just alienating to trans-binary people, I think it is literally dehumanizing to all GNC people. That's why I pointed it out.