It's once again (like the output menu) poor design, a waste of screen space, and confusing to boot.
I would expect to assign the effects block to a footswitch rather than assigning the footswitch to an effects block.
The minimum expectation is to arrange the buttons in a row so that you don't have to scroll unnecessarily.
It would be better to have an assignment button under each effect block where you can assign the footswitch (I-IV).
Can't you assign multiple fx slots to each footswitch? Then the proposed layout doesn't make much sense to me.