Just because a test joint of unknown size and shape held up over that range doesn't mean an actual real world assembly can hold up over that range. We know not

Just because a test joint of unknown size and shape held up over that range doesn't mean an actual real world assembly can hold up over that range. We know nothing about the nature of the weld they tested or the nature of the testing. Even with two of the same materials it's very possible to design a joint that doesn't work very well in the real world.