nowhere in any height velocity diagram description does it say that autorotatios are not possible from whithin that area.
the height velocity diagram depicts an area of flight where an autorotation may not be carried out successfully, meaning without damaging the airframe, etc. etc.
demonstrations of all kinds of autos in this area are not difficult to carry out.
it is possible to do hovering auto's from two hundred feet (when you know they are coming). the extra height allowed in most diagrams takes into account the fright factor.
mostly it comes down to degrees of experience and the environment you are operating in.