It's fine for, say, a blog post aimed at Haskellers to assume Haskell knowledge, but when posted on a board largely consisting of people without Haskell knowledge, it's natural that you're going to get at least a few people saying, "hey, I don't understand this".
But I'll be honest - I'm familiar with Haskell and the ML module system and the underlying concept (that typeclasses and modules are roughly equivalent in some sense), but I'm unfamiliar with Backpack so I still struggled to follow it a little. The target audience is an extremely narrow niche. So I think it's just somewhat poorly written; it doesn't feel like the author really had an audience in mind, other than themselves. There's probably ways of writing this - without spending too much time regurgitating the basics - that would be more palatable to more people.