In my understanding the OR rule indicates that one of them MUST happen. It cannot be that both cannot happen. It's either M-J or J-N. One of these two must happen.
A or B but not both means one must be in at the exclusion of the other.
... as either M-J or J-N but not M-J-N, so ... : N-M-J, M-N-J, J-N-M, or J-M-N.
It can't be N-J-M because the rule states ... that J must be either after M ... the case in the N-J-M ordering. It has to ...
Here's the master guru J.Y explaining this question. It's a tough one, and it seems like a lot of people thought that the first sentence was the conclusion.