Earlier quoted context omitted.
As far as I know they still use the Darwin kernel in iOS. Which means it scales from desktops and servers all the way down to phones... That seems pretty general purpose to me.
No one uses Darwin for servers, not even Apple.
You can turn old laptops into "servers", or raspberry pi's, or hell even a commodore 64 for that matter. As long as it serves something of value over a network it qualifies.
Whether its a good idea is another matter.