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.
New Mac Pros/Mac minis actually have OOB management features. However depends on having an MDM and another Mac on the same LAN I believe.
The fact that Server.app has been gutted and gutted for years now.
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.
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.