Few arguments against it:
- Pressing the flag button doesn't mean actually submitting it (as in every test/form).
- There might be an unknown duplicate, if you assume it's the first post (as you were told) it might be worthy to flag it.
- You might want to look if someone else thought it was duplicate by "making half way" to flagging it.
Why pressing the flag button makes you fail an audit? I think it shouldn't.
The implementation difficulty (fake flagging UI) and the tips about how avoid failing (and reviewing!) by skipping the answer adds some insight, but still doesn't answer the question why it's not a valid review.
Moreover, I would like to add one more, this question describes a reviewer that failed an audit while trying to flag a question as off topic. The question was clearly off topic, and still after 1300 views no one flagged it.