I don't see anything in the GP about being offended. And I totally agree; I automatically and almost unconsciously take an article less seriously if it's got glaring and common errors.
I also disagree with your characterization of the amount of work that went into the blog post. From the number of screenshot examples and the overall structure of the piece, it looks to me like the result of a significant amount of work.
That said, direct feedback (like through the links provided at the bottom of the article) is probably a better way to let the author know about (and correct) the mistakes than a comment on HN.
I like Stephen Fry's take on grammar and 'correctness':
This looks to me like a typical personal blog where a non-native speaker jotted down his notes about a certain topic. The text is perfectly understandable and the grammar mistakes are harmless. Take it or leave it.