UKL is multi threaded, not multi process because there is no fork. Plus it's a Unikernel i.e., just one process. You don't need any scheduling code, Linux does it like it does normally for kernel and user threads.
Slight clarification. Yes, UKL has normal kthreads, but all the application code runs using pthreads. And their implementation is unchanged, except the fact that they do function calls instead of syscall.
Will Serverless End the Dominance of Linux in the Cloud? https://dl.acm.org/doi/pdf/10.1145/3102980.3103008