ohh ok I see. the difference is that on RDL, after you go down, you go up fast and explosive (standing all the way up). With stiff legged deadlift, you go slower speed all the way through.
Why does that site say that RDL is very hard to do? it is very easy to do. It is just that the knees go slightly bent so the butt can stick out (with back straight [flat as in when bending over, a drink can go on your back without falling off] the whole time without arching too much like those incorrect example pictures), and then stand back up fast with back nice and straight with explosiveness