They contend Hoff Sommers legitimizes “male supremacy.”
The SPLC wrote (emphasis mine):
The men’s rights movement has a dedicated international following, including in the United Kingdom and in Australia. Women, too, have helped give the men’s rights movement a veneer of even-handedness. Prominent MRAs [Men’s Right Activists] also include anti-feminist female voices, such as popular Canadian YouTube personality Karen Straughan, American psychologist Helen Smith, and the former head of a domestic-violence shelter for women, the British Erin Pizzey. Men’s rights issues also overlap with the rhetoric of equity feminists like Christina Hoff Sommers, who give a mainstream and respectable face to some MRA concerns.
Guessing how SPLC apologists will respond, this excerpt doesn’t say that Ms. Sommers is associated with “male supremacists”. Instead, it seems to be calling her a useful idiot, in that her rhetoric can be abused by “male supremacists” in support of their ends.
Rather the way a group can abuse the mantel of “anti-hate group” to demonize people they disagree with.