why didn't the chap start immediately after saying it? Why wait 10 seconds for someone to walk into danger?
Well, it is a judgment call how long one should wait. Of course, starting immediately would not give anyone who might be close to the prop enough time to get clear (see generally the link supplied in my earlier post). On the other hand, waiting long enough for someone to casually stroll over to the aircraft is probably too long (especially if that person was originally so far way that they couldn't make out the "clear!" call).
I don't believe it happened. If you heard someone shout something from an aircraft, would you walk over, through the prop arc, to find out what was being said? Nor would anyone else.
I also find it almost incredible that someone would approach a single engine aircraft from the front, knowing full well that it was occupied (and therefore likely to start up). However, I don't doubt that it happened as described. There are plenty of stupid people out there; and even normally prudent and intelligent people are entirely capable of occasional thoughtless acts.