Article by IEEE Spectrum: The writers tested the two AI image generators MidJourney and Stable Diffusion, testing their abilities to generate imagery that closely resembled copyrighted material, which proves that the training data of the image generators had to contain copyrighted material. Implemented safeguards were largely unsuccessful to curb the output of potentially infringing images.
Also this doesn’t prove that they used copyrighted materials because there are exceptions to copyright & gray areas like fan art.
They admit that they did though.