It’s when the buds unfold right? I honestly can see how there’s some subjectiveness to it.
I don’t consider budding trees making the canopy turn a slight color as leaf out… I want to see at least some decent opening of the leaves.
I feel like folks judge it on a spectrum, like fall foliage.
“Leaves are changing” in the autumn to one person may mean you saw a red tree along a swamp. To another person, it’s all still green. “Leaf out” in spring to one person may mean some green buds mixed with sticks with one warm spot seeing some opening. To another person they are waiting for widespread green and opening of buds.