It's an operating system that runs server software. How does that not make it a "server OS"?

It runs a VPN service, an update service, it's largely based on OpenBSD, how are you defining what is and isn't this magical "server OS"?

Incidentally, I didn't actually need to purchase that, it just made my life easier. I was quite readily able to install OpenVPN on it via Homebrew and configure the pf rules without their GUI.

The only thing I couldn't do was to install the caching update service. Do you know of an alternative I could have used to update a network full of Macs? No mockery needed, just an answer will do fine - if you know.