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.
If you read that article, they tested two unikernels and the key point was that unikernels don't implement security mechanisms present in a normal OS. This Linux Unikernel, otoh, does not delete any of that security functionality. The boot up code is the same, it just calls a specific code instead of bringing up the general userspace. Unikernels and security don't have to be mutually exclusive.
Will Serverless End the Dominance of Linux in the Cloud? https://dl.acm.org/doi/pdf/10.1145/3102980.3103008