They claim they have faith
unlike a true non believer who would
Not even claim they do
If they claim to have faith and do not (do not have eternal life) why does James not teach them why they really do not have spiritual salvation.
What is more consistent and logical is to see that James regards them as having
received eternal life and but have not put that gift to use, and he is telling them that there are benefits
for the soul to put one's faith to use.
The claim to spiritual saving faith is REAL, that is not the problem James is addressing at all!!
The problem is claiming faith but not putting faith to use (inoperative, separated aka dead faith) in the living outward what they possess inwardly.
This is the correct view and holds true how the word "dead" is used in scripture within that culture.
Making
dead mean
not real or only
mental assent is not how the ancient people thought.