In SpriteKit, it's possible to move nodes by directly setting their velocity property.
This doesn't seem to work in SceneKit, however. Setting velocity -- both in game loop callbacks and outside of game loop functions -- seems to have no effect.
This answer from one of the Apple SceneKit engineers suggests accessing the velocity property is only possible during a game loop callback like updateAtTime.
So clearly there are limitations around the velocity property.
Is controlling the property directly not possible with SCNNodes?
As @Confused kindly pointed out, the docs show it is possible to set velocity directly: https://developer.apple.com/reference/scenekit/scnphysicsbody/1514757-velocity
The key is to use a dynamic body type, not a kinematic one (and obviously not a static one).