I think you are misunderstanding what the author means by "stock Debian." The issue here is that the hardware is so unique that we need to add kernel modules that are not included in the standard Linux kernel that ships with Debian. So we need a custom Linux kernel. Close to stock might also be referring to any config changes the author needed to make in order for the switch to function properly.

This could be done with any Linux. I'm not sure what 'complexity' you are referring to, but the complex open modular nature of the Linux kernel allows us the flexibility to build a custom kernel for this interesting application without unnecessary modules. To be frank all modern operating systems are complex because modern hardware is complex and user expectations are high. Some just do a great job of hiding the complexity from the user. Linux makes an attempt to expose it's complexity, but it's complexity is about the same as iOS, macOS, Windows, etc.

If you wanted to do without Debian or any distribution you could compile your own kernel, your own user space tools, and then install it on the hardware. At this point updates would need to be applied by pulling code from upstream and compiling it yourself. Look into Linux From Scratch to get an idea of how Linux without a distribution works. However, I don't think the juice is worth the squeeze in this situation.

EDIT: My answer to "is there another OS", to which the answer is yeah, probably a dozen. I think a blog post of doing this with openBSD would be interesting because I'm not sure what the exact steps are to install custom drivers on a BSD and openBSD has a very high standard for security. I think the reason it's done on Linux is because there is a lot of expertise about Linux and this type of project is relativity straightforward with Linux.