So the library does not depend on OpenCL version or does?

The library does NOT depend on the version, since it supports pluggable engines. Currently the only available engine is focused on AMD GCN GPUs and uses OpenCL 2.0 features in its kernels. Other engines are possible and planned, notably one that would work with older versions of OpenCL and support tuning for Nvidia.

Going by the last paragraph in the article, the library currently supports "OpenCL 2.0" only, but "OpenCL 1.2" support is planned.

Not really directly related to the OP (sorry), but I have always wondered how the hell people manage to remember the right xkcd. Awesome :)

Does it work on NVIDIA GPUs?

As soon as NVIDIA support OpenCL 2.0 in their drivers, which may be this year and is technically possible, but depends on their strategy to promote CUDA. But, you can write OpenCL 1.2 code with ClojureCL and it will work on NVIDIA. Or, if you need 2.0, use AMD...

They recently updated their drivers and now support OpenCL 1.2 on recent GPUs.