I haven't yet seen code-only answer that could not be improved by adding some explanatory text (have you?). As such, code-only answers are almost perfect candidates for downvoting:
Downvotes are the community's way of telling peers that their content can be improved...
But (and this is a very very big BUT), in support of your point, note that code-only answers are officially perfectly legal. Neither FAQ nor How To Answer give any indication that these are somehow prohibited.
For comparison, look at the strong discouragement given there to link-only answers - "provide context", "may be removed" - there's nothing like that for code-only answers.
The most negative evaluation I could squeeze from above references so far was clicking the adapted from link in "How To Answer", getting to Answering technical questions helpfully (note - this is not a part of SE network) and finding this statement somewhere in the middle, in a plain text, without any special emphasize:
Code without an explanation is rarely useful, however. At least provide a sentence or two to explain what's going on.
Summing up, all the authoritative references I've seen
have no trace that would even remotely hint
that deletion of code-only answers is encouraged - none, zilch, nada.
Because of above, I believe that suggesting deletion as a valid review option for code-only answers would be unfair. Decline to recommend deletion or choosing "Looks Good" for a code-only answer should be considered valid outcome in a test for stuff like this. If this is not the case, either test has to be redesigned / removed, or an authoritative reference should be provided to back up such an expectation in test.
For the sake of completeness, I personally agree that code-only is not a reason for deletion but that's not quite relevant here.
Side note. To me, the issue you describe also indicates a problem in review system design. I made a simple test to simulate actioning downvoting-candidate answer and I believe I figured what's wrong there.
- Review suggested me an answer that I decided to be worth downvoting, not deletion - just an action I mentioned as most likely appropriate for code-only answers.
- After some fussing with UI, I discovered a tiny link to an answer, clicked it (to open in a new browser tab) and downvoted - so far so good.
- After that, I returned to review window. It was showing me the same set of options - looks-good, edit, deletion, not-sure. Hm.
- I refreshed the window, my shiny new downvote was there, great. Now what about my options?
- Oops there are the same looks-good, edit, deletion, not-sure.
Hey! Hel-lo-o-o-o!! didn't I just actioned the item suggested for review?
Didn't I just put my own precious reputation where my mouth is? downvoting answers is not free, you 'know
Given that I downvoted, what of these freaking options suit me? None "looks good" now, every one sucks in its own way.