Hi ashpine,
I'd recommend reading my earlier post (Post #37) on this question. In that post, I break down how the argument works and what each sentence does in the argument.
It can be found here.
viewtopic.php?f=552&t=4065&start=30
Specifically, the last sentence in the stimulus and its role in the argument are discussed in this part of my post.
"We then get another premise that states that many of the most realistic artworks are NOT the best, which is the opposite of what would be true if beauty equals truth.
Given this new fact, then using the contrapositive, it must be that beauty does NOT equal truth, which is the conclusion of the argument (i.e. there is a difference between beauty and truth)."
So the short answer is that the final sentence in the stimulus is a premise, and it is a critical part of the argument for the reasons that I provide in the earlier post.
Hope that helps.