Your flag was declined because while they were a new user there was no indication that they were affiliated with the product being mentioned and as such did not meet the flag comment which states
- Exists only to promote a product or service, does not disclose the author's affiliation.
The actual content of the deleted answer (10k rep link) was
I did Googled nircmd.exe and it's referring to this webpage: http://www.nirsoft.net/utils/nircmd.html which is a tool for "Edit, merge and fix the AutoComplete files (.NK2) of Microsoft Outlook."
Surely this is not for controlling the brightness steps of Windows.
Also this is not an hardware issue because the 1% brightness steps can be done within the Control Panel's settings. Surely a software design issue, please use Windows's feedback hub to upvote similar issues / submit your own.
It was not advertising. It was primarily commenting that the description for the tool, which itself was mentioned by the already existing answer, on its website is not describing a solution for the problem that was asked. As it was nothing more than commentary it was flagged by other users as "not an answer" and was deleted as such.
Your flag was declined so as not to unnecessarily feed the spam filter with stuff that isn't spam. Spam flags apply several penalties as mentioned in the answer to the question you linked and there was no crime beyond posting commentary as an answer.
What are the “spam” and “rude or abusive” (offensive) flags, and how do they work?