It's both. The biggest hole so far has been resources. SPM didn't have a way to define them for applications/libraries/tests. You can't create many interesting GUI applications without images (e.g. no application icon).
We just got bundle support for Linux. A proposal for SPM resources is apparently being worked on, and will be coming in the fall. That means it will finally become practical to define and build GUI applications using SwiftPM.
The Xcode team's goal is to improve the developer experience on- and for Apple platforms. I'm sure they would love to implement native integration with Swift's official package manager. It's not going to stop people editing, building or debugging their applications in Xcode. I find it surprising that anybody would suggest they might have some kind of pathological hatred of SwiftPM.