A simple scan of the repo's description and the first sentence of the linked article should answer that question. LKL is a library that allows you to do Linux things (read/write linux filesystems) from outside of Linux, essentially re-using the kernel's code. UKL, is an unikernel which essentially allows you to compile an application into a standalone OS that runs it (not sure if that's entirely correct).

Now that I think about it a little more, they're kind of similar but I think unikernels are more of a way to get an application to run on hardware without all the extra bloat of a monolithic kernel (like if you're running a web service on a small virtualized machine) and the LKL is just for re-using Linux code.