This side of the paddock does seem a bit odd, the other side is large stones and hot wire, not sure why its been done like this, but yes in theory the giraffe won't step over the barrier, while the oryx, and the variety of free ranging animals can access the moat type bit
giraffes as a rule (in theory) don't step over things that are beyond a certain height, they also don't like uneven surfaces hence the stones which could also be an effective barrier if it was just for giraffe
Like said above, they do have a certain height, can you imagine a giraffe lifting its leg over such a height!
It is to do with the angle of their necks and therefore the posistion of their heads, they are unable to judge heights and distances, i.e. ha ha's / haw haw's properly as their heads are over the objects before their feet and therefore are at risk of tripping over and breaking a leg, at least thats one theory.