None of the claims in this have anything to do with the essence of security, they all address the window-dressing of security.
What does it mean for something to be secure? As I see it, for something to be secure, it means that every security invariant holds under all attacker models.
Let's break this down. What's a security invariant, and what's an attacker model?
A security invariant is a statement like "information may only be read by principals authorized to do so" or something similar. The blog post talks about "oh well every exploit I've seen pops a shell with /bin/sh and we don't have /bin/sh so you can't hack it." This attacks the symptom and not the disease. If the attacker is in a position to run /bin/sh you are already at a disadvantage, the attacker was previously going to break your security invariant by spawning a shell and then copying out your secrets. They could instead just use their code execution to read the secrets directly, ala heartbleed, if they are in the same address space.
An attacker model describes the attacker, their motivations/goals, and capabilities. For example, what if the attacker wanted to take your unikernel enabled app and repurpose it into an e-mail spam bot? Well, there isn't a writeable FS and there isn't the ability to exec stuff, so the old model of "copy some files up and run them" won't work. However, is there executable memory in general? Can the application allocate existing executable memory? If the application is, or involves, for example a modern JavaScript interpreter, then it has to be able to do this in some capacity. So now the attacker modifies their methods so that their goal can be achieved in the constraints of this new system.
There is an argument you can make about sandboxing and principle of least privilege, but that argument transcends unikernels really. Look at the sandboxing and separation strategies used by programs like chrome, vsftpd, and qmail. They (in conjunction with enforcement technologies like seccomp-bpf or selinux) can make the same guarantees about the lack of system calls and least privilege.
I'm pretty sure that the title of this post really should be "unikernels are obscure" because that's the layer of protection you're really getting here.