SpriteKit can, as of El Capitan, use Metal, not OpenGL, as a rendering engine.
Syphon does not (yet?) support Metal.
SpriteKit typically defaults to using OpenGL Core Profile behind the scenes.
Syphon does not yet support Core Profile in the main release. There are however forks of Syphon that can get you there.
As for integration, you are likely going to have to make a custom SCN or SKNode that makes its own renderer,and implement syphon there. Likely it should be a root node and render its children prior to evoking some sort of texture copy phase to nab the contents of the current rendered buffer.