The reason "you already knew" is correct, is that the sentence example, "The manager told us more about the New Year's party", is past tense, and when the tutor explains that it means "you already knew", he's talking about what your state of knowledge was at the time you were told.
If the tutor said "you've already known", then the focus (present perfect) is on your current state - even though of course it includes a continuous state of knowledge from before being told, continuing all the way until now. Because he's talking about a point in the past, it would sound a little unnatural here.
However, the tutor could definitely have said "That means, you had already known something about this plan" (past perfect, instead of present perfect), which keeps the focus on the point in the past (when you were told something "more"). That would be completely natural, and could probably be argued to be more correct - but in practice it's unnecessary here. It emphasizes that the speaker had a continuing state of knowledge... but that information isn't really important, and is somewhat implied by the word "already".