Joseph stated the halacha 100% correctly. He never actually said they don’t have to honor parents, just that they maintain no halachic tie, which is true. This comes up all the time when it comes to ownership of a dead, unmarried ger’s property. How anyone with the most basic knowledge of Gemarra could call Joseph out as being wrong here is beyond me. And, yes, his CR reputation does precede him; if by that you mean he keeps it interesting and has kept a lot of us coming back for years.
I think the OP’s question would have been more interesting had it been regarding a goyish grandparent with a typically Jewish name. Say the ger had a grandpa David.