They are not used in preprocessing, I mean that the backbones (e.g., ResNet, VGGNet, DenseNet...) pre-trained on ImageNet dataset are used as the initialization of the synthesis model in training stage.
As mentioned in your previous forum question, the pre-trained models are not allowed to be used because of fairness. I quite agree that private or public task-related datasets and pre-trained models cannot be used in this competition.
However, the backbones pre-trained on ImageNet are widely used in neural network training, and these pre-trained models can be easily obtained by everyone. So I don't think that using these models will affect the fairness of the competition. But I still respect the decision of the organizers.