My understanding is that unikernels are single-application by design. I agree that a mechanism to spawn other apps is useful, but in this case it won't be a simple fork but the creation of a complete new os instance.
I suspect GP is referring to single applications that aren't single process. Postgres is one example. Vacuum and checkpoint are separate processes.
Applications also sometimes have separate processes for sentinal functions, logging, etc. Or default configurations that are multi process, like many load balancers, web servers, API gateways, etc.
