My thought about HDPE being food grade is that there are not plasticizers or residue of precursor chemicals (e.g. Bisphenol A (BPA) left behind in polycarbonate), etc used in HDPE production or molding. Nor am I not concerned about low-toxicity contents that are highly soluble (drain cleaner, alcohols, etc) because I wash all containers throughly . I wouldn't reuse a container that had held high-toxicity materials like pesticide.
But if you know of a formulation of HDPE that isn't food-grade, please post or send me a citation.