I suppose it is through the static electricity - when the liquid flows through the tube - the risk of spark-over. Since I can not imagine what nauseating liquids to be transferred in some form thus excluding fuel, and the product for drinking water probably is not appropriate (interesting smell, PVC, plasticizers violently?), And almost all Vorrezensenten have so happily pumped fuel, I suppose, the Note is only a safeguard clause against possible claims for damages.
I have purchased in good faith in order to pour kerosene from canisters in fuel tabs, and doing this on your own risk. Less risk-tolerant minds will resend or dispose of the product. I think the marketing concept with the coyly incomplete product description is not in order. As should surely at least on functional limitations also drinstehen, then what is in the instructions.
I would therefore not even emptying my tank, so a blue-eyed question to the seller: Which liquids, only a concrete example is enough for me, the thing really is?