That's only for doors opening across the bottom of the stairs.
As there's no specifiic rule against a door over a single step, it is permitted. It would be wise, however, to hinge the door on the newel side so that you don't open it to find yourself standing directly over the next step down.
We tend to get hung up on stair dimensions of treads, risers, pitch etc, but its all advisory only. The only regulation about stairs is reg K2 which states only that stairs shall be 'safe'.
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