In my opinion calling the code garbage is much nicer than "The author of that post has a common misconception", which sounds like a passive-aggressive way to call someone dumb. Everyone writes bad/'garbage' code once in awhile.
Not at all. It's a polite way to say they're ignorant of a particular thing. It would be rather insecure of them to read into it and think they're being called dumb across the board.
When you say "the person who wrote this code has a common misconception", that's a statement about the person. You've jumped to a conclusion about the reason the person wrote the bad code. You're telling the person to accept that they don't know as much about mutexes as you, that they're ignorant, that they're a worse programmer (albeit in a fairly narrow category).
This is why I think GP's wording comes off worse even though it's superficially more polite.
Additionally, _sometimes_ offering someone an out to save their ego actually does them a disservice.
Giving someone an out is a great thing to do, especially for people for whom saving face is important. But I don't think "this code is garbage" does that. Something like, "that code is measuring timing incorrectly because X", conveys the same information without being inflammatory. "Garbage" is a value judgment, while just pointing out that it's doing something incorrectly is merely stating a fact.
Saying that stuff at the beginning only clutters the reasoning, as one should not "just believe" that that code was "indeed garbage", just because he said so.