The book of James contains doctrinal truths for the Jews living in the last days of the tribulation as they will be scattered and will face all kinds of trouble.
1 James, a servant of God and of the Lord Jesus Christ, to the twelve tribes which are scattered abroad, greeting.
The term twelve tribes is used all throughout Scripture, and is always a reference to the nation of Israel as a whole and never to a called out group from that nation. Genesis 49:28, Exodus 24:4, 28:21, Ezekiel 47:13. And the NT is no different: Matt 19:28, Luke 22:30, Acts 26:7, Revelation 21:12. There is no justification for taking this as anything other than an address to the twelve tribes of Israel. In order to make James a Christian epistle would mean to contradict every other usage of this term in Scripture. It is certainly not a reference to the diaspora.