It's just a 'technical' way to say 'the car must be full' and not just 'look like its full'. I doubt they'd every push anyone erroneously on this topic who legitimately filled up.. because anyone with any sense would simply call and complain to corporate and have the charge reversed.
Imagine if you are driving a truck or van.. 10 miles can be a half a gallon or more. By driving this point, it's just a scare tactic to get people to buy the pre-paid gas options.
Low cost rental agencies have been notorious for decades on trying to nickle and dime people. It used to be on scaring people into buying the insurance... then it was charging people for trivial damage to the car that may or may not have been there before... now it's the gas options... Many people avoid the B or lower tier rental agencies JUST to avoid these kinds of tactics. Desk staff can be instructed and driven to really press these topics with their quotas, spiffs, oversight, etc.
This thread should be "dollar rental at MCO was very pushy with me about buying the gas option" - End of Story.