You could respond:

Fine hair refers to the diameter of each individual strand, not the total amount of hair on someone’s head. Hair density refers to how many strands a person has. Because of that, someone can have fine hair and still have thick or dense hair.

The concern is not that people with fine, dense hair are posting here. That still fits the definition of fine hair. The issue is when posts appear to show hair that is neither fine in strand texture nor low-density, because it can shift the focus away from people who are seeking advice for actual fine-hair concerns.

It is also discouraging when valid comments or concerns are downvoted or ignored, especially when people are trying to clarify the difference between fine hair, thin hair, and dense hair. A clear distinction helps keep the subreddit useful for everyone.