That's no longer true. The Foundation framework is basically complete on Linux, and the vast majority 3rd party libraries which are not iOS specific will work on Ubuntu. Even many of Apple's own libraries (i.e. SwiftNIO, a low-level, high performance networking library for things like implementing web-servers) are cross-platform.
You'd be surprised. There are a few reasonably well developed server-side frameworks, some of which are already used in production various places.
Also a lot of the libraries which are mostly used in iOS don't have any dependancies on the iOS platform: for instance promise or event emitter implementations etc.