I think that page of the documentation is wrong.
If I set PlayStart, and then use the roVideoPlayer it buffers to that point and starts playing, and then if I press left on the remote it allows me to go back in time towards the beginning. I can start playing from anywhere. That's how it's always worked for me with video in this case. Probably it's getting confused if you have both set. Try setting BookmarkPosition=null before adding the content to the videoplayer.