Depends on the concentration found but Synephrine itself is a naturally occurring substance which can be found in quite a wide range of plants.
I recall there was another positive recorded to this exact same substance somewhere in Australia in 2011 or 2012 and if memory serves me well because it was shown to be widely present in plants growing in the pasture that the horse grazed it ended up being dropped.
Unless they can/could quantify the amount present and categorically state that it was above anything that could possibly be caused by the consumption of a plant or plants that contain it then I would think it would be near impossible to make a charge for that stuff stand up.
Maybe the horse has a taste for Manderins?