In the absence of an 𝘢𝘤𝘤𝘦𝘴𝘴𝘪𝘣𝘭𝘦 email account used to sign up, the only suggestion can think of is that they try to find a old device they used for Imgflip, might still be logged in on that device. Somewhere recent I saw an old user account saying they got back into their Imgflip account this way.
If that works, carefully change to a new email in
imgflip.com/settings and reverify the new email.