"This guy is a member of "The Nation of Islam" which simply is not considered to be a truly Islamic organization."
Which authorities have decided that, and why do they continue to be counted as Muslim, in population reference sources.
The Orthodox Muslims.
and why do they continue to be counted as Muslim, in population reference sources.
For the same reason that Mormons are counted as Christians when most Orthodox Christians do not consider them so. It is how they consider themselves that counts in that case.
One is a civil label that is self awarded and one is a religious label that depends on the majority of the religious group you are claiming to belong to accepting that claim.
This leads the field open to folks to make up all the numbers they want about anything they wish.