So it appears that you can’t import a file without it being enclosed in a rectangle object that’s created for it (unless you drag’n’drop stuff?).
But it also appears that with the API you can set an ImageFill on nearly any Graphic scenenode.
Is that all correct?
Nuanced explanations welcome.
I would say that’s correct (not a lot of nuance to go into 
). I believe (although I can’t test that right now) that you can also use CtrlAltV on Windows or CmdOptV on macOS to paste an image from the clipboard “into” a selected layer as fill, but that basically is the same as drag’n’drop.
Apart from that, GraphicNodes simply act as “containers” having a shape, fill and stroke, where the fill can be an image. Therefore, there is no more “real” way to import an image, it’s, when you not drag it onto an existing shape (with a different shape/ different dimensions), just a rectangle (for obvious reasons, as it can always match the bounding box/dimensions of an image) with the image as fill.
All in all, you are therefore correct and this is as much nuance as I could possibly get into this answer 
Thanks for confirming!
If any Adobe folk want to add nuances, that’d be helpful, too.