Yes, I included the kernel in my list of things that are desirable to scale well for that reason.
That said, in some cases I don't think it's strictly necessary for even the kernel to scale well as long as you have a hypervisor that does. It's not unusual to deploy software in VMs on a cluster. Having more, smaller VMs per machine is a way to handle poor kernel scalability, just as I suggested for the web application server. VMs are higher-overhead than multiple containers on a single kernel, so this wouldn't be my first choice, but many people use VMs anyway.
That said, in some cases I don't think it's strictly necessary for even the kernel to scale well as long as you have a hypervisor that does. It's not unusual to deploy software in VMs on a cluster. Having more, smaller VMs per machine is a way to handle poor kernel scalability, just as I suggested for the web application server. VMs are higher-overhead than multiple containers on a single kernel, so this wouldn't be my first choice, but many people use VMs anyway.