I learned
why the Knights say "Ni!". Néa must know this already. "Ni" is the Swedish for "you" in the plural, and used to also be the formal singular "you", like the French "vous". However, Swedish added a strange twist to this. If someone was eligible to be addressed as "ni" then, by definition, it was inappropriate to do so. Instead, one referred to them by their title, e.g. "would the professor like another cup of tea?" As the writer concludes, "the world really is coming to a sorry state when people are going about saying "ni" to old ladies."
I suspected the story of being a wind-up, but I happened to have a copy of "Teach Yourself Swedish" dating from 1969, and it agrees. It also said that the awkwardness of having to address people in the third person, while also avoiding the words "he" and "she" (taboo in this situation), had led to efforts to persuade the Swedish public to use "ni" more widely, with some success. However, the success must have been limited, as I understand that the formal "ni" is now all but extinct.
It is not know if the Knights who say "Ni!" are a deliberate reference to this quirk of Swedish grammar.