I’m thinking that “lifetime” makes perfect sense - since for many people the end of the transmission’s life makes the end of the life of the vehicle.
So you apply the same logic to oil, your oil is also “lifetime” fluid. You never have to change it because this will kill the engine and then the oil will have lasted the lifetime of the car.
Or, in other words, the entire “lifetime” fluid thing is ridiculous and does work very well for sales & marketing but is terrible for car owners.