YC will have to make a discoverable false statement of actual fact. They're unlikely to publish a list, and what they do reveal to YC founders is unlikely to be phrased as simply as SPLC's statements about Nawaz.

I think this is less likely to be legally complicated than you think. Consider: YC has been doing this for years already.

Not even the same sort of thing. I hope Maajid Nawaz wins, because SPLC has abandoned universal ideals and has become short sighted and partisan. However, it's also within the rights of people and organizations to associate or not associate. It's not like they're a bakery. Their business isn't to sell a cake to any presentable customer who can afford one. Their business is to select and foster winners in starting businesses, and staying clear of controversies is certainly germane to that.

EDIT: When it comes to conferences and professional organizations, I think those things are more like a bakery than like YC. A programming conference is in the business of fostering discussion of and dissemination of information about programming. A code of conduct is fine for such an organization, but a politicized code of conduct has no place in such an organization. There's something clearly wrong with politicized code of conduct in a non-political organization that makes a subset of its members uneasy. There is no place for intolerance in a liberal humanist multicultural society, and people who want to use the bureaucracy of non-political organizations to alienate their political opponents are intolerant people who have given up on convincing people. Such people want to bully people into doing what they want. What they are doing is only the softest form of intolerance.