The main use case for unikernel apps (the way I see it) is running language specific VMs like Beam, MRI or the JVM almost directly on bare metal and getting rid of all the complexity of OSes. The idea is to make it easier to debug, optimize and tune applications by removing traditional OSes complex kernels from the equation. The real argument for security (that the author omits) is derived from that: 20 million less lines of code in the stack that you deploy.