I don't see where they say this. I see that they say you can upload your own stable diffusion images and that you can use images from lexica.art as inspiration.
""" You can upload previously created stable diffusion images. Consider the awesome https://lexica.art website for inspiration. The images from there can directly be used as upload images on this website as well. """
They suggest heavily that the prompt you give is running stable diffusion and that it is your unique image on the that is generated
See this image https://www.thisshirtexists.com/wp-content/uploads/2022/09/S...
which advertises the following text
""" Specify your subject
Generate Art through AI
Create your unique shirt now!
Order your unique shirt! """
This strongly suggests you are getting a unique image from your prompt not a nearest match from somebody else.