Generally, the accusation of "sexist" is used when someone is making unfair or irrelevant assumptions on the basis of sex. It is not generally leveled at technical documentation that assumes the reader has the technical background to read the documentation, for instance.

Technical documentation sometimes does contain humor, it's not that uncommon for developers to include jokes and easter eggs. Furthermore, this is listed under category "Poul-Hennings random outbursts", described as "You may or may not want to know what Poul-Henning thinks.". So while technically included in it, it's not really a documentation of anything. It's basically a blog post. And just like I don't find anything wrong about a blog about cosmetics or fashion assuming that the reader is a woman, I don't find anything wrong in a blog post about technology assuming that the reader is a man.