The system automatically selected that answer for use as a review audit because it was deleted after being flagged as spam or rude or abusive. I know this because 10k users can view deleted posts, but posts that get one or more "red flags" before being deleted have their bodies hidden until one clicks through to the revision history:
In this case, the flag used was almost certainly spam. The post looked like spam because it recommended using a website to extract a .7z archive, which is something that can be done easily and far more conveniently by just installing the necessary software on Ubuntu. Using a website to do this is cumbersome because you have to upload the archive, then download the contents after they are extracted. Apparently this was helpful to the author, but it is not a reasonable approach for the vast majority of people, might not have been reasonable even in their case (they may simply have been unaware of other ways to do it), and the post didn't explain why one might choose to use that method.
In hindsight, this post was not very likely to have been spam. Examining the contributions here and on other Stack Exchange sites of the user who posted it reveals that they're unlikely to be a spammer, using websites to perform operations on files is not categorically unreasonable, and the answer did contain this useful caveat:
If your data is not very sensitive (though the webpage claims to delete the files immediately), then the above online option works great.
That doesn't prove the post wasn't spam. Spammers can write stuff that looks like they're thoughful and critical, too. But I don't think the author was affiliated with the website or that it was otherwise posted with spammy intent.
It is also not really a link-only answer. A link-only answer points to somewhere else where there is claimed to be an answer, rather than answering the question itself. An answer that says "use this program," and links to where the program can be downloaded is not a link-only answer, and this doesn't really seem too different from that. The fact that the service it recommended could go offline is one of several reasons to consider the answer low-quality, but it doesn't make it a link-only answer in the usual sense, at least as I understand that policy.
I don't recommend that this answer be undeleted. If I saw that answer, I would downvote it, comment to express the criticism of it that I've given here, and probably even vote to delete it, which 20k users can do. As written, I would consider the answer to fall under the second bullet-point for when it is acceptable for members of the community to vote to delete answers:
The answer is extremely low quality: There is little to no scope for improvement
However, if it turns out that there is substantial disagreement on this and the answer is undeleted based on a consensus that it is valuable, then I would definitely respect that and refrain from attempting to redelete it. (If it is undeleted just to clear the spam flag and prevent it from being used in audits, however, then I would comment and vote to delete as described above.)
Should you have failed that audit? I think really you should not have failed it--that is, that your actions there fell within acceptable reviewer behavior--and that it would have been better if that answer had not been used as an audit. However, anytime you see an answer that looks that much like it might be spam, I recommend that you click through to the question page where the answer appears and try to figure out if it is part of a pattern of spam. Doing that would also have revealed that it was an audit.