And Jehovah's Witnesses are unique in their outright rejection of the New Covenant, the one that offers forgiveness of sins.
The definition of Christian is someone that believes in Christ, it is not someone that accepts the New Covenant. That is just your interpretation of what is important. A Hindu/Muslim would have no difficulty recognizing a JW as Christian, it is only Christians that are splitting hairs to differentiate themselves. I see your definition as nothing other than a marketing ploy.
Jehovah's Witnesses reject the Blood of the Covenant for themselves personally. This is fundamental to what being a Christian is.
Who says acceptance of the covenant is fundamental? Scriptures such as Acts 16:31, John 17:3 shows it is belief in Jesus.
Further, JWs do accept the New Covenant, they just believe it is mediated to them through the anointed. Just because the 144,000 have inserted themselves as an intermediary does not mean the New Covenant is rejected. I disagree with this Watchtower doctrine, but it does not mean they are not Christian.