They don't seriously mean it as canon, they're just making jokes about their wishful thinking, because there was a thing in the avatar magazine that teased at the idea (which also doesn't count as evidence) and because in the game Zuko kidnaps Katara. Which is more like saying that the plot premise of fanfic is now canon, not actual Zutara being canon.

Sometimes a few members might have thought that there was canon, if they misread people's reactions and jumped to conclusions.