Why do you think it's stupid? Here are my reasons for the statement you quoted:
1. It's been years since anyone has emailed me an .rar attachment. In fact, I don't think I've had a legitimate .rar file in my Inbox since the turn of the millennium.
2. The last time I regularly opened .rar files was when I was into warez. Those things often contained malware.
3. As a result of 1) and 2), I would be very suspicious if somebody sent me an .rar file.
Of course, that's just my perception, so other people might associate the .rar extension with better things.
These are exactly the sort of stupid statements that make so many people frown on the HN community.