No, you did not do anything wrong. You reviewed the contents of the post, thought it was not suitable as an answer, and clicked according to what you thought of it. This is the desired behavior of reviewers.
It happens that the post was an answer to a very difficult integral, where giving a closed form with no explanation is still a major contribution. One can test the correctness of answer numerically beyond a reasonable doubt, and knowing the form of the answer helps in finding a way to its eventual proof.
(Historical example: according to John Stillwell, Mathematics and its History, "after hearing that the sum [of $\sum 1/n^2$] is $\pi^2/6$, Johann Bernoulli himself discovered a proof, which turned out to be the same as Euler's".)
As I wrote in Review FAQ
if you are reviewing carefully, an occasional failed audit is of no real consequence and should not discourage you from reviewing.
I add an explanation from higher up:
the primary purpose of audits is education not punishment. It's easy to make mistakes, especially after you've been doing a fairly monotonous task for a good while - sometimes, it's helpful to have a sanity check. I fail audits occasionally myself, and always take it as a sign that I've perhaps gotten in too much of a hurry and am in danger of becoming careless; the audit (and countdown timer that goes with it) is a welcome reminder to look up and take a breath before going any further.