That simply isn't true, based on what happens in the movies, as well as what would realisticly happen.
In ESB Luke hits Vader with the saber, and doesn't cut off anything. Same thing happens in AotC.
And finaly lets look at this reastlicly, or as realisticly as we can considering we're talking about a lightsaber here.
The only way a lightsaber can cut off a limb or someone in two, is if enough of the blade passes though the target. A grazing hit with the tip of a lightsaber would not chop off anything. The lightsaber may be a energy blade, but it is still has physical dimentions, and as such works the same way any other blade does.
If I swing and hit someone with the tip of my saber, at the end of my swing, it would only be a grazing strike and wouldn't chop off anything, or do much damage. To cut someone in two for example, the blade would have to extend at least a tiny bit past the back of the persons body, and the cut would have to have a arc long enough to pass from one side of the body to the other. Simply tapping someone in the side with a saber isn't going to cut them in to.