was presented with a page telling me the author had deleted the answer
I clicked no action
So the site stated "there's an issue, please investigate", and you had seen the issue was resolved, so replied "it's sorted" (no action needed).
In one sense it was perfectly logically to do so, and to be honest an audit for a user removing their own post is perhaps a little unfair.
If it was this review, you could argue the answer was perfectly fine, and so your review of "no action needed" was acceptable. The author deleted their post for reasons other than it actually needed deleting.
However, reviewing with "no action needed" is like stating "this answer is fine", and the site knows the answer is deleted and so thinks you were wrong. As did you, really ;) (you'll know for next time)
You can usually get around any niggles with the review being an audit by just continuing to review based on what the review outcome should be.
So in your scenario, this is where you should realise it's an audit, but even if not, as you knew the answer was deleted you follow suit and downvote, or flag, or click delete, etc.
The audit system is not perfect, and sometimes shows things like you have found. Also, for example, you can see in the review area an answer has 0 votes, but viewing the answer in the question it has 12 upvotes.
You know it's an audit, and so click "no action needed", upvote, etc.
While this all might sound pointless, the fact you went to the question means you are paying attention. You are investigating, and looking at the scenario.
Essentially, you are not just coming to the review and without doing anything at all clicking a random outcome (robo-reviewer).
And it not being perfect does mean it catches us out sometimes. Many of us have been tripped up.
There is not much more can be done to make sure users are at least doing a little bit of investigating ("paying attention"). Not without complicated scripts and algorithms etc, and to be honest, with millions of users it's likely not worthwhile.