It could have been a simple mistake. Perhaps someone meant to click 'valid' but ended up clicking 'invalid'. Though we're extremely careful, it is easy to make a mistake when doing a repetitive task. We don't have the ability to go back with flags, once dismissed they vanish from our tools. Any other action we take can be undone with just a few clicks.
Typically, a moderator will mark a flag as valid if they are able to see how or why you might have formed your opinion of the post in question, even if they don't take the action that you suggest. We much prefer the community to take action when it comes to borderline questions.
Additionally, we tend to take no action, even dismissing, if we're not familiar enough with the topic at hand to judge the technical merits of a post. While most flags we service require us to examine the quality of a post, NARQ requires us to know enough about the topic in order to decide if the question is answerable in many cases. Sometimes questions are just downright incoherent, but we come back to quality there - not evaluating if enough information has been provided in order for someone to answer.
With that being said, I'm pretty confident that this particular case was just a mistake that happened as a result of a human being doing repetitive work. I'm pretty sure I'm not the one that dealt with your flag, but I have made the exact mistake that I'm describing several times in the past.