> I don't understand the appeal of trying to strip the Linux kernel down, then introducing additional specialized code like nolibc

The appeal is you can eliminate every single dependency. Linux is the only kernel with a stable user space interface. It's the only system where you can do this.

With a single 6-ary system call function, you can do anything you want on Linux. There's no need for anything else.

> What's the upside of using the (relatively large and complex) Linux kernel for this?

The kernel's massive amount of features and drivers for one.

> I doubt it's reliability, since the kernel in this particular configuration is essentially untested.

This is not a kernel configuration though. User space is completely separate from the Linux kernel.