Could anyone please explain how a Boeing 767-200 came to be flying at 546mph (NIST),575mph (RAF)?

Could anyone please explain how a Boeing 767-200 came to be flying at 546mph (NIST), 575mph (RAF) at 700 feet altitude? Boeing insists the wings fall off at 440mph at this altitude. Surely one would need 6 engines not 2 as the air is 3 times the density at 700 feet as it is at 35,000 feet. The retired flight director of NASA, Dwain Deets recently commented 'that is the elephant in the room that nobody is speaking about’. Has no one heard of ‘Equivalent Ground Air Speed’? Thank you.