Q:

A flagged post was declined, how do we deal with what may be deemed as "incorrect"?

Hmm okay, I know moderator's actions are final, but are we allowed to question their decision?
Case in point is HERE
Now, I flagged that as very low quality (Another had already flagged it as NAA), but I decided it was probably low quality. It seems, after moderator review, that the answer remains.
It was declined as the usual:

declined - a moderator reviewed your flag, but found no evidence to support it

I mean I know flags are based on opinions, but surely the majority of the vote goes towards that answer being very poor quality and verging on not even an answer?

A:

There were two flags on that answer. A Very Low Quality flag, and a Not an answer flag. It's not VLQ, but it is also not an answer.
I declined your flag. Here is the answer in its glory:

As you said the MediaRecorder can not split the ES while recording. If so, could you share your way that how did you derive the ES while recording.

I declined it because at the time I read the answer, it did look like an answer. 
Looking at it now, it is part answer (affirming what the OP already knows), and part question "share your way that how did you derive the ES while recording."
Reading it over again, I don't believe it's a answer, so I'll delete it.  
It's not Very Low Quality. Had your VLQ flag been the only flag on the answer, I would have declined it.
Very low quality has a specific meaning: The post is so undecipherable that no amount of editing will correct it.

A:

The flag was probably declined because it is not a very low quality post. Use those for posts that do answer the question, but are neigh unreadable to the point where you have no idea how to edit into anything sensible.
This post is perfectly legible and can still be edited to be a more complete answer. I would agree with the Not An Answer flag, but a moderator can only respond to multiple flags with one response.
The moderators are treating Very Low Quality flags quite strictly these days, so the choice here was to reject your flag. I am a little surprised it was not at the same time converted to a comment though.

