At risk of being downvoted, myself, as this is off-topic, but since there is no 'meta' and you're clearly looking to understand what happened, I suspect the issue originates in the first statement made from the original comment, which was posed in the form of a question:
Didn't you read the rest of the article?
My rule of thumb is that if a question can have ", moron?"[0] appended to it and
both the meaning and tone remain unchanged, then it's going to be taken as condescending. You'll probably find that people who are particularly sensitive to keeping Hacker News a "civil place" (among which I count myself) will read that sentence and skip the rest of the comment before hitting the downvote button. Had it been left off, the rest of your comment offered clarity to anyone who simply skimmed the article or otherwise wasn't understanding what was being presented and might have missed that point. Removing the sentence also doesn't affect the information being conveyed[1].
While I'll take you at your word that it wasn't meant as condescending, a variant of Hanlon's Razor[2] applies which I had shared with me as "Where there are gaps of understanding, people tend to jump to the worst possible conclusion." and this serves as a pretty good example. You assumed the reader didn't bother to read the article, when it was equally likely that they simply didn't understand what was being presented -- a fair estimate since this is C++ templates, which is not something everyone encounters in their day-to-day. And folks who are able to downvote likely did so because they assumed you were being condescending, when it's equally likely that you may have simply written the comment hastily or meant the question in a less-sarcastically sounding manner -- a fair estimate since verbal queues are missing from written text and most of us type near the speed that we think.
[0] Or "dumba$$" or "?, God!" (followed by a gesture of hands being thrown down in disgust)
[1] I once had a great manager who, at a yearly feedback session, told me rather bluntly that I am too verbose and worked with me on how to convey a message by learning what is unnecessary for conveying it. As you can tell if you glance through my comment history, the lesson was entirely lost on me. While I'm much better at eliminating things that add negativity to my intent (often by adding more to my statements), I'm pretty terrible at identifying when brevity has higher value than precision and completeness. Though, when it really matters, I can pull it together if I take enough time... and on and on and on.
[2] https://en.wikipedia.org/wiki/Hanlon%27s_razor