I followed the instructions in their README, they suggest conda, but don't tell you to use it.
My GPU isn't even CUDA-capable, but the previous version of Upscayl worked fine, possibly on the CPU. That might have changed, I don't know.
In any case, even with conda installing everything would have been a pain to someone not really accustomed to the Python ecosystem. I don't care too much about the GUI, but a working AppImage is really nice.
What GPU do you have? The Upscayl README says in big letters:
"NOTE: Upscayl does not work without a GPU, sorry. You'll need a Vulkan compatible GPU to upscale images. CPU or iGPU won't work."
Sure, perhaps a previous version may have worked without a GPU, but this seems surprising given that it uses a GPU-only library for all the heavy lifting.
My GPU isn't even CUDA-capable, but the previous version of Upscayl worked fine, possibly on the CPU. That might have changed, I don't know.
In any case, even with conda installing everything would have been a pain to someone not really accustomed to the Python ecosystem. I don't care too much about the GUI, but a working AppImage is really nice.