I once was standing on a deck while the the top of a door to be planed was resting on the ground a few feet below (to plane the bottom of the door).
You might have noted that Paul Sellers does not necessarily works vertically or horizontally.
See for example how he planes the ends of the work-top and aprons of the workbench. (see introducing picture of the video “workbench: episode 1” [the solid wood one])
You could also have been standing/kneeling down on your bench.
See 2nd picture in “will you apprentice with me?” dated 4 August 2015; although I would not have done this for that box.
And John’s piece doesn’t seem to be clamped except maybe with his foot and the piece against the wall. (it probably works better with a very sharp plane).