Neh, I'm made the same "mistake". Must either be a language barrier or a shortcoming of language itself. I read it as that nUI5 must be disabled if it's present :-p
Perhaps a better way would be to not mention nUI6 at all in the sentence. Something like
nUI5 must be installed but inactive.
Not that it matters, I'm sure Spiel will have a full 6 beta soonish