The article talks about OS capabilities in the second part when it discusses Mojo, which is based on IPC.
> The solution is to design a language such that if I import leftpad, then call it, my computer can't get hacked.
That requirement may seem clear right now, but the moment you talk to other people about your language you'll find there's no agreement on what "get hacked" means. Some people will consider calling exit(0) repeatedly to be "hacked" because it's a DoS attack, others will say no code execution or priv escalation happened, so that's not being hacked. Some will say that left-pad being able to read arbitrary bytes from your address space is being hacked, others will say no harm done and thus it wasn't being hacked. The details matter and you need to nail them down in advance.
It turns out for example that one of the top uses of the Java SecurityManager was just to stop plugins accidentally calling System.exit() and tearing down the whole process. It wasn't even a security goal, really.
> You stop components tampering with each others' memory by using a memory safe language.
That's not enough. See languages like Ruby or JavaScript, which are memory safe but not sandboxable due to all the monkeypatching they allow.
> Haskell already passes a type object as an argument to anything which does IO. They don't do it for security. Turns out having pure functions separated from non-pure functions is a beautiful thing.
But almost nobody uses Haskell, partly because of poor ergonomics like this! So if you want a language that gets wide usage and has a good library ecosystem, monads for everything probably isn't going to take off.
> If nobody created a writable file handle, the file can't be arbitrarily mutated by another module.
We're talking about critical bugs in the filesystem so what the FS processes idea of a file handle is doesn't really matter. If you can confuse or buffer overflow the FS process by sending it messages, you can then edit state inside that process you weren't supposed to be able to access, and as that process controls the security system for everything it's game over. Microkernels have no way to stop this, which is one reason very few operating systems move the core FS out into a separate process. You can't easily survive a crash of the core FS code, and it being exploited is equivalent to an exploit of the core microkernel anyway in terms of adversarial goals. So you might as well just run it in-kernel and reap the performance benefits.