French Lilac is toxic in its raw form so needs processing to neutralise the toxins. It is not available as an extract or timcture, but is manufactured into the form that we use today. This is why we say it is derived from the plant, but is not a supplement currently available to apothecaries / naturopaths for folk medicine such as migh be sold on Amazon. A similar statement would be made for Digitalis, which is derived from the foxglove flower, but which is also toxic in its raw form.
There are many plant based medicines that we use today, and which have been processed from the raw state to remove toxins. In the olden tines of folk medicine, the apothecaries would do this processing, but those skills have been taken over by the chemists and pharmacists to provide cheap and easy to use potions that have quality control applied to ensure safe dosage and repeatability. I believe that we no longer use Witch Doctors here in the UK but some may use that terminology for derogatory comment.
To answer your post more directly
http://www.diabetesforecast.org/2010/dec/the-origins-of-metformin.html