There is no reason HLLs can't be used in kernel space, is there? Excepting shell-like languages (which require fork, which you typically can't do in a kernel), most embedded kernels provide everything a HLL might expect in kernelspace.
"Virtual memory—which gives us the user/kernel space split and the process model of programming whereby programs are protected from each other—is just the most pervasive form of isolation."
That's not claiming virtual memory is the only reliability benefit, and you can't expect a short article to be a treatise on all the differences between kernel space and userspace.
Furthermore, the article isn't claiming that kernel programming is _better_ than userspace programming, it's just pointing out that there's a tension between performance and safety - which you acknowledge!
Mind you, most high-performance software doesn't need to go this far, and stuff like PF_RING / Linux' packet_mmap or "batched" systems calls like recvmmsg() (or even read()) further reduces the number userland/kernel transitions.
That is not a good thing if you care about performance.
If I want to move the TCP/IP stack to userspace on my Linux machine, how do all the programs I use (firefox, ssh, IRC client) send and receive packets without context switches?
If your answer is to change the userspace architecture and shove everything into one process, you've made isolation _worse_. If you don't, how do you avoid the context switches to talk to the network stack?
The key point is that the network stack has to own shared state, and sending and receiving packets _must_ touch that shared state. If it's accessible to every process (shared memory), you've tossed out isolation and security. If it's not, you have to context switch.
Processes are one way of doing isolation but they're a very crude one (give each thing that needs to be isolated its own address space, swap the whole stack every time you switch tasks). A userspace scheduler (which, sure, would be running within a single process at the UNIX level, but that's the wrong level to be thinking about it) that knows about the details of what it's running can achieve the same level of isolation in a more lightweight way.
Are you going to claim they do networking faster than OpenBSD?
How fast does OpenBSD do radio networking on mobile phones?
How fast does OpenBSD do CAN networking on car systems and factory installations, with real time constraints?
1. It entertains a performance argument without interrogating the task and environment the appliance is engaged in. You can always in theory make a typical program faster by rewriting it in carefully scheduled assembly. We don't often do that because the performance win would be irrelevant to the problems we're trying to solve.
2. It pretends that the only reliability benefit from moving to u from k is in virtual memory. But, no: the major benefit is from moving from C to something more expressive and safer. The notion of "defect density" captures this argument: with it held constant, a solution that can be expressed in fewer lines of code will have fewer faults. But, of course, defect density between C and a high-level language won't be constant, and it won't favor C, where you can't even count safely with the basic types provided by the language.