The first illustration shows how this works in tmux. The illustration first has a split in left and right half (vertical split?) and then a split in top and bottom (horizontal split?) of each of the halves. So if you resize one of the panes vertically, it only affects the other pane in the same half of the window, while if you resize horizontally all the panes will be resized. Basically vertical resizing only resizes the inner split, while horizontal resizing resizes the outer split.

It seems Zellij somehow makes the order of the splits irrelevant.

I think you're right, I was wondering the same thing.