And how come you assume that even if said person indeed has the XX chromosomes (to be nominally classified as a "woman"), that they also self-identify as a woman and prefer the adjective "she" over "he" or some other?
See how far this rabbit hole goes?
It's easy why they assumed the author is male: they don't care who the author is, and didn't read their name on the blog. I read the first 6-7 paragraphs of the post and didn't bother to read the name either. And even if I had, "Sabine" as a name doesn't tell me anything about the gender (not a name we have in my country, nor a very well know one, like Mary or Helen or something that everybody would guess). Heck, there are Spanish men called "Maria".
Had I made a comment I might have used either "they" or assumed it's a man, mostly because statically for things posted at HN, it would be true.