Mature adults should be able to take criticism without getting butt-hurt about it. The real world can be mean and nasty and sometimes you need a wake-up call that you're being silly or screwing up.
I have seen a lot of comments on Hacker News that I thought were mean. This isn't one of them.
Rounding is something I learned in elementary school. If someone in elementary school was unfamiliar with rounding, it would be mean to take them to task about it. But this writer is an adult, a professional programmer, and presumes to be a teacher, by taking the time to publish an article about the subject.
In fact, the writer has even less excuse, because he knows about rounding. Twice he mentions "rounding down". So it is all the more baffling that he did not call what Apple was doing, simply, "rounding up". Instead he wrote a multi-paragraph blog post about adding 500 milliseconds of "fake time", then having to duct-tape over that by making the timer say "0" when the timer reached 500 ms.
Suppose a man bills himself as a car salesman, but every question you ask him about the car, he says the wrong thing. He says the car has 3 wheels when it has 4. He says it is red when it is black. He says the engine is in the back when it is in the front. Would it be mean for you to mention his title, by saying something like, "You say you are a car salesman, but you don't know about this car on your lot." It is right to call someone out for being less than what they say they are.
As for random blog posts on the internet, this isn't so surprising. What's surprising is that it wound up on Hacker News with over 100 votes.