justify follows the flow, align crosses it; column turns the axes.

A flex container has a main axis, set by flex-direction, and a cross axis at right angles. justify-content always works along the main axis and align-items always works along the cross axis. Switching to column makes the main axis vertical, so justify then moves things up and down, as long as there is spare height to move in.

Read it once, then a question tests whether the trick stuck.