Well, that comes down to what you think a server is. macOS is definitely unix, and can run server software. You can do that on that rackmount mac. So what makes it not a server?
servers generally will have out of band management, redundant power and networking, some kind of light you can flash to help find it in a rack. stuff like that. oob management is probably the most vital feature of a proper server.
Looking at the back, it is missing things like redundant hot-swapable power supplies. The processor is one oriented toward workstations rather than servers. Not certain if the OS can be configured for huge pages in RAM.
Talking about the hardware here. Yep, Server.app is a shell of what it could be. But I mean the rackmount macs — I was asking GP why they didn't consider those servers ;-)
Not GP, but because they aren’t marketed as servers and don’t have features you’d commonly expect in a server. Video and audio production heavily uses rack mount gear and so having a rack mounted version of a Mac Pro makes perfect sense.
