> But it's not even an essay, isn't it?
I was using the term "essay" because its definition is so vague that almost any short not so academically strict non-fiction text may be called an "essay", at least in a wide sense. (And you can even call a rather austere text that deals with a broader subject an "essay", like Hume's Essays.)
But I am open for a better alternative: So if we have a short non-fiction text that wants to make a statement in a more casual style, how would you classify it instead?