The funny thing is, the Legalist form to apply here: https://www.legalist.com/equifax/#apply-form

points to the Equifax page here: https://www.equifaxsecurity2017.com/potential-impact/

As far as I remember, you could enter any random information into their form and it will say you were affected. That is the proof that Legalist is asking to provide the basis for the suit.