So you are saying Hazrat Ali R.A. was not a Muslim so that his followers can't discuss Islam ?
you know when you talk to christian they say muslim dont believe in jesus( hazrat isa a.s), while this is not true we do believe in hazrat isa but not as a son of god but as a messenger of god. because we don't believe in hazrat isa as christian do. so they say we don't believe in hazrat isa.
same is with shia, they think that we dont believe in hazrat ali. we do believe in him but as a companion of muhammad pbuh , we dont elevate him to the status equal to prophet or allah. he was a great companion of prophet. but he was not equal to the dirt of muhammad shoes. he was just human and even prone to mistakes in his life.