At a lower altitude, the air density is lower, and falling objects are accelerated faster. Conversely, the air density is higher at a higher altitude, and falling objects are accelerated slower.
Flat-Earthers deny gravity and invented the “explanation” that an object falls only because it is denser than the air surrounding it. But if it were true, an object should fall faster at a higher altitude where the air density is lower, but it falls slower instead. This fact disproves flat-Earthers’ density theory.