They do update the model in the background, although I'm not sure how often or how much they update it. To avoid issues with this practice they offer gpt-4-0314 which says this in the documentation:
"Snapshot of gpt-4 from March 14th 2023. Unlike gpt-4, this model will not receive updates, and will only be supported for a three month period ending on June 14th 2023."
Unfortunately this experiment is using the frozen snapshot model gpt-4-0314 instead of the unfrozen gpt-4 or gpt-4-32k models, so any differences are literally 100% noise. This would be a somewhat interesting experiment if someone were to use an unfrozen model, though. I do appreciate the author for captioning the images with the exact model they used for generation so that this bug could be caught quickly.