I agree with Matt here insofar as "Class reductionism" isn't a concrete descriptor for an argument but a political insult/accusation built from a large possibility of arguments. Someone saying all oppression originates from class oppression is a "class reductionist" but also someone saying "the only oppression that matters is class oppression, the rest is just idpol bullshit" is a "class reductionist", and calling someone that is less describing their argument and more accusing them of being the latter in a rhetorical move to force them to defend themselves before their arguments are even faced properly.