I do believe it, because it happened. Look it up, it's a fact.
A single time, a member of QI copied an addon that had a backdoor, modified the addon, but left in the backdoor. He then released the addon, and, again, as far as i'm aware, the addon was not presented by Team [QI].
To say "Team [QI] is known for making backdoors" for that is a stretch at best and lie and worst. People certainly don't know them for it.
In fact, if you didn't bring it up so often, most people wouldn't even know about or remember it.