They aren't your teacher. They aren't trying to send the content to you. They are just blogging on their own website for their own audience.

And its hardly unique to this article. If you are writing about the nitty gritty of linux networking, you probably aren't defining what TCP or UDP means. If you are writing a super detailed article comparing and contrasting plot structures of different animes, you probably aren't going to start by explaining what the word anime means. Etc

I'm not saying the world should be all RTFM, but if you are reading some sort of specialized content, then yes i think its a reasonable assumption that the reader has some basic background knowledge on the topic at hand, or is willing to do the research themselves.