Hi SLF,
There are a couple different issues cropping up here, so let's take a look at all of them. I'll start by explaining this question from the angle in which LSAC sees this problem.
LSAC would make the point that there is a difference between essays and authors. Meaning, it is possible for a single author to have written multiples essays in the book. Thus, when you break down the stimulus, there is an opening there so that only one of the three authors is selected (and not two, as you thought). Before looking at that, let me just say that it is a tricky word game they play, but perhaps not unexpected since this is question #19, and #19s are often tough (not always, but some of the hardest questions out there have been #19s—probably because this is the point in the section where many people begin to feel serious time pressure).
Let's look at the stimulus more closely. The first sentence shows plurality on both essays and authors: "An editor is compiling a textbook containing essays by several different authors." This could mean one or more essays by each author are included. Note also that this simply says "authors," and the reference in the next line mentions three specific authors, but based on this sentence, there could be more authors in play. Thus, when the next sentence says, "The book will contain essays by Lind, Knight, or Jones...," it's possible that just one is selected, but that multiple essays from that person are included. This is how they get past the "essays" objection you raised, and it then cancels your question about the Uniqueness Theory applying to the two answers.
One way around this issue is to realize that under your interpretation of the scenario—that exactly two from the group L, K, or J selected—there are multiple correct answers (A, D, and E are all correct when exactly two are selected). The rules of the section would tell you that is impossible, and thus you would have to go back to the stimulus to re-evaluate your analysis.
Final note, one issue you ran into in your analysis was that this statement is misleading: "L
K or J." LK is ruled out by the stimulus, because if you choose L and K, you then get J, which violates the stimulus statement that you can't have all three.
Tricky no doubt, and annoying as well.
Please let me know if that helps. Thanks!
