Oh please don''t tell me that you (the anchored) are being found at fault !...Thats ridiculous!
Here is another question you should pose back to the Coast Guard and attorney. What if the dinghy was on a davits? Then what if it projected off the back of your boat? Then what if the dinghy was partially on the back of your boat? (I sometimes lift the front of my inflatable onto the sugar scoop transom) Where does your anchored boat end and an officially recognized tender?
BTW How much damage was done (to both your dinghy and they guy who hit it)?
One thing about Marine liability is its rarely 100 % one persons fault or another. They tend to split liability, though in this case if you were more that 5% culpable I would freek.
Maybe you should have grabed your neck, started screaming and checked yourself into the closest hospital with whiplash....;-)