How can you defend that one. It sounds like a someone had a bad day at work. It looks like a scrap from the cutout for the closet flange fell into the tank.
Second issue sewage grinders, I have a problem with them because of just this type of incident. What do you do when the pump grinds to a halt because of a foreign object in the tank (Grand kids toys...), you are in very big trouble.
I'm glad it worked out for you and you are correct in that you should purge all tanks the first time out. This would include sterilizing the fresh tank.