Judaism is neither an ethnicity nor a racial group, yet people are allowed to identify as that being a disadvantage of some sort in a lot of contexts.
Regardless, the problem with atheism is that it falls under the same category as being white. There is not enough of ancestral history to support any sort of urm. Little history of slavery, discrimination, etc. Our grandparents have had "too much of a good life" according to medical school for us to complain. Same with atheism. Yes there were times where not believing in god would have got your killed, but that was never on a large enough scale. Basically, not enough people have died in the name of atheism, for it to be considered an urm.