I don't think Facebook would have been credibly able to rate “not the primary cause” as false - that requires being able to quantify the individual contributio

I don't think Facebook would have been credibly able to rate “not the primary cause” as false - that requires being able to quantify the individual contributions of each factor. To rate “not caused by” is false, they just need to argue it contributed to the slightest extent: it's a much lower bar.