You opened up a review post in order to get better context and make an informed review. That's great, please keep doing so.
The problem was that you took a review action based on the current status of the post, which isn't what you were supposed to review. The review queue asks for judgment on posts as shown on the review screen. Clicking "no action needed" equates to saying "this post is valid". In fact, there's a tooltip to that effect:
If you click "more", you're shown this:
I agree that the "no action needed" language could stand a bit of improvement here. As it is now, it's just a tautology which gives no additional information to the reviewer about the meaning of that action. The tooltip isn't nearly as discoverable as the "more" text, so this is a case where user experience could be improved. Something like the following would be better:
No Action Needed when this post has no issues which need addressing.
However, the existing language doesn't imply that you should click "no action needed" if a post is already deleted, since you're supposed to be reviewing what's shown to you.
Well, it wasn't obvious to me. It's obvious once you know it and then it's easy to hack the audit system.
Audits aren't meant to trick you, and they are intentionally obviously easy. The point of audits is to ding people who just click through reviews as fast as possible, without actually reviewing it, in order to grind badges. That, and educate reviewers who make outrageously bad judgments - which isn't you, since you made the correct decision about the content. Deleted posts are ideal for this purpose.