Since my polymorph meme has only garnered three downvotes so far I thought I'd offer a bit more controversial take, and see if I can manage to stir the pot a bit with this one.
It specifically makes a point of saying "can siphon life from others to heal your wounds".
So, Self is a valid target for the damage. However, the heal doesn't occur because the target is not someone else.
If self was valid for the siphon there would be no reason to include the "from others" rider