I can tell you that one you put weight on the system, you do not need to have it over a limb, around the trunk is just fine.Im probably over thinking it looking at it...just seems funny to me for such a simple system attatchment that unhooks so easy could be a safe anchor to descend from..watching a few rappel videos and they always seem to do a back up anchor point.I'm not trying to bash your system in any way. Have you had any slippage using it around the trunk only or do you always pick a tree with a limb as a stopper to keep any slippage from happening...?
Looking at the photo, it does look very odd to me. I am sure there is some sort of side load being applyed to the biner in that position from the tree. I don't think biner's are intended to be used like that, I use a screw link because I know they are ok.