What might it have been?

Why wouldn't the author say what it was, if there was such a thing?

Read the "tl;dr" part at the end of the article. (Someone used the word "eugenics", and the author was trigegred by this word.)

Why [do you believe there was most likely one specific thing that offended the author]?

The contrast between the widely general text of the article, so that I had problem finding out "what exactly is this person trying to say?", and the very specific "tl;dr" at the end, which I really would have problem to extract as the main point from the article.

My model of humans (which may be more or less correct, but this is what I use) says that the most likely way to this outcome is the following:

Author reads something and becomes angry. In their mind this connects with dozen other things (which weren't actually present in the article). Author decides to write a reply in the form of article in their blog. Author writes very generally, because they want to show how things are connected; to emphasise that the thing that triggered them is not just a trivial detail, but a part of an important whole. Overwhelmed by emotions, author writes too general article, and only at the end notices they actually didn't write the thing that triggered them. But the article is already finished, so they add a "tl;dr" at the end.

A competing hypothesis would have to explain why the "tl;dr" is seemingly unrelated to the rest of the article, when in fact it should be a summary of the article. (My hypothesis is that it is a summary of what author had in mind when deciding to write the article.)