The title of the thread is unquestionably sexist and dismissive;

I don't see the title as particularly sexist. I'd agree with it generally even. See my replies to dahlia221.

I could discuss my experience of dysphoria, and being a menstruator, and then if I decide this makes me have authority on the topic to dismiss one life experience for all others -- that is an example of sexism.

How do you know those people consider themselves an authority on the subject? Because they're talking about their life experiences and the conclusions they make don't agree with the conclusions made in the GC thread? That's why I'm calling sexism on your part. You're assuming that they're coming from a place of internalized misogyny rather than just sharing their experiences (thus the I'm not like other girls example you use). If an AFAB doesn't think periods are as bad as X thing, doesn't agree with certain aspects of feminism, doesn't identify as a feminist, finds "womanhood" to be problematic, etc they're reasoning is immediately dismissed as internalized misogyny. It's assumed that these AFABs are "trying to please men" rather coming to these conclusions on their own via their own ideology/life experiences. It's pretty degrading to have your own thought processes reduced to "you just want to please a penis" or "you just hate yourself."

learn new ways to talk about their pain without needing to put natal females down in the process

Why the emphasis on trans people when many in that GC thread are straight up insulting trans people and justifying it with their menstrual experiences? That's a typical double standard applied to minorities. Cis women can say demeaning things in their own space and it's all peachy but when trans people share their experiences in a space intended for that, simply sharing that experience is morphed into a misogynistic appeal to authority (which is hilarious in its own right because trans people have virtually no authority).