The knock, if a lie in red based on a
Definite Description error, is debatably possible to speak about in red. I used the "Present King of France" trick jokingly in Battler Solves the Logic Error to have Battler talk about aliens in red when (more than likely) there are no aliens of the type and in the place that he described. Going by the rules as established in ep5, however, I think this sort of wordplay is red-compatible.
There is an argument to the contrary by various philosophers, but it's a complex question of formal logic and reference. The question is, what does "the knock" refer to when used by the characters who talk about it with authority? We presume as a matter of course that these statements refer to a specific existent concept, but as the letter debate sort of shows that isn't
necessarily true.
However, if we divorce "the knock" from any physical action, how exactly can we talk about what it was or was not at all? If it's not a reference, what is it? If it's a symbol, are some of the things Lambda said about it actually safe to say in red?
It's probably not a question Ryukishi got too deeply into given his philosophical shallowness in other areas. A bit hypocritical, I think, coming from someone willing to employ semantic trickery.