Well, I'm a leather hobbyist ... ...I wouldn't want to wear boots made out of full-grain 15 oz hide from adult steers anyway.
But of course when the hide comes out from the tanning pit or drum, it would be full-grain. I was just assuming the leather would have been split after tanning to achieve a thickness that's more suitable for boot-making? Or the CXL used on the boots come from younger animals or thinner cuts on the hide?