On your 'flag summary' page, ab2, check to see whether the reason that the moderator gave for declining the flag looked like this:
declined - flags should only be used to make moderators aware of content that requires their intervention
—because that's the message I received in response to my VLQ flag on the very same question you ask about.
Maybe a mod got annoyed that we flagged the post instead of marking it for deletion (although it was deleted just 11 hours after posting). Or maybe the mod was just having a bad day. Or maybe there has been a sea change in mods' tolerance for flags that don't "require their intervention."
I had always imagined that when an answer garners enough delete votes to trigger deletion, any flags associated with the answer switch off (meaning that mods don't have to deal with those flags at all), and that flags remain live only when too few delete votes are recorded to delete the answer within some arbitrary period (say, a week). If that's the case, I don't know why any moderator should have been made aware of our flags in this particular instance, since that should have happened only if the answer were still live after the arbitrary period had elapsed—unless we had flagged the answer for spam, rude/abusive, or a customized reason, which we did not.
Flaggers have also received advice from moderators at various times to flag answers that don't seem salvageable, as for example in these comments by MetaEd to Why was my VLQ flag rejected?:
Consider that a Very Low Quality flag is a flag for deletion. But a wrong or unreliable answer is still valuable to readers because it attracts downvotes. A downvoted answer warns a reader “don’t go this way”. A deleted answer does not have the opportunity to warn readers of anything. Also, moderators have been told we are not arbitrators of whether an answer is wrong. That decision is left to the whole community by way of up- and downvotes. Deletion is for answers on which a downvote would be wasted: answers that are not even wrong.
In short, if the answer is right or wrong, reliable or unreliable, vote it up or down. Otherwise, if unsalvageable, flag it.
Since I can't both flag a bad question and vote to delete it (unless I leave the review queue), I have often flagged rather than voted to delete VLQs and NAAs. That way, if the votes cast for deleting a bad question don't reach its threshold for deletion, a moderator will see the flag for reviewing the still-live answer after it has been up for a week and can judge whether to delete it anyway. On other occasions, I jump to the original answer and both flag it and vote to delete it; however, if you try to do this in the wrong order, you'll find that you can't do both.
If the moderator's reason for declining our flags represents a shift in policy by EL&U mods as a group, I will submit fewer flags in future. My intention never was to make work for moderators; it was to reduce the likelihood that bad answers might slip through the review process unscathed.
In a series of comments below this answer, Janus Bahs Jacquet clarified what must have happened to the VLQ flags that ab2 and I left on the original post in question. In the event that his explanation gets deleted for some reason, I will reproduce it here:
The flags we're dealing with here were definitely declined by a mod, not a queue: the reviewers unanimously voted to delete, and a decline reason was given, which is only an option for mods. But that reason makes no sense whatsoever for a VLQ flag since those do not require moderator intervention.
It would be quite out of character and completely pointless for any of our mods to decline a flag that requires no mod action with a statement that flags should not be used to make mods aware of things that do not require their intervention. The only logical conclusion is that the message was [not] in response to ab and Sven’s flags, but to a “requires moderator intervention” flag which we obviously cannot see.
I remember mods saying in the past that when declining flags, it is not possible to do so individually: all pending flags are declined at once; so declining an incorrect “requires moderator intervention” would automatically also decline ab and Sven’s correct VLQ flags. [See "Disputing flag reviews".]
The upshot is that the moderators weren't annoyed at the VLQ flags from ab2 and me in connection with this post, and their standards for finding such flags helpful haven't changed. Instead, the VLQ flags that ab2 and I submitted got bundled with a flag that required a moderator response—and when the moderator rejected that flag, the other flags on the same post got rejected, too, en masse.
Thanks for solving the mystery here, Janus!