If CAG's normal policy is to lock threads that assert information that's known to be false or has no reliability behind it, but then they waved this policy for this joke rumor, it's not really a knock on kotaku.
Threads are locked here when the OP provides no evidence or there is no reason to believe them. If this didn't happen, gaf would be less reliable. Obviously it looks bad for kotaku to react the way they did, regardless of whether they're right or not.
If I'm lied to, and then I go tell somebody else the lie without looking into it, I should have checked into the information before passing it on. Especially if that's my core responsibility. However, "well you should have known I was a liar" isn't a good defense for the liar either.
disclosure: I don't read cag or kotaku.