Your concern probably reflects peer concensus, although the HBU analogy is specious because HBU analysis is prescribed whereas research into the potential impact of flooding is one of a million factors that might affect valuation, e.g., fire zones, earthquake zones, Superfund zones, etc., etc.
My perspective about the relative awareness of a potential buyer--regardless of how theoretical the point was meant to be--goes to "market reaction," which as the basis of the Sales Comparison Approach to appraisal valuation is by definition based upon the collective perspective of market participants; and if a theoretical buyer isn't aware of, or concerned with, flood information when making their "informed" decision, why would that factor be taken into consideration when cumulative market reaction is determined? And if not, why would it be included in an appraiser's analysis (for the sake of argument of course).