Identitarian is someone who believes in identity.
If something ends in arian, it's a kind of person.
Vegetarian is another example.
"-ism"s are ideologies like capitalism or communism or socialism.
Vegetarianism is the ideology that vegetarians follow.
In the case of capitalism, the people who follow it are called capitalists., have communists and socialists as well.
We could have called vegetarians vegetists and their ideology, vegetism but for some reason, we went with the other one.
It's two suffixes that sort of mean the same thing.