This sounds slightly out of context but I would imagine the message here was more along the lines of "you can call a thing which a poster says as being a stupid thing to say, but you cannot call the user stupid"
Exactly. People can say things that are stupid and pointing that out isn't an insult to them. Saying that somebody is stupid is an insult. There is a clear difference between the two.
I have to say that it doesn't really encourage respect to call someone's point stupid in general, even if it's not an insult it isn't compatible with the theme of respect. At least when curbing respect, that should be discouraged until respect and intelligence is restored, then it might be interesting to leave some room for poking fun at each other that way. But it would be a priviledge earned by the community building respect.
It's like you can't make fun of your friend until you've built enough credit with him to feel free to pull his/her leg a little.