That seems to imply it is somewhat image independent, but not based on the content of the image but rather the camera/lens/f-stop combination?
That's correct. Not the image content/subject matter dictates Capture sharpening, but the capture process does.
For example if you are using a pretty small aperture, knowing you will get some diffraction, wouldn't you modify the capture sharpening somewhat? I assume this is the best place in the work flow to try and correct diffraction.
Absolutely. Here is the result of an analysis I made for a specific Camera/Lens combination, and it shows how the required Capture sharpening radius varies over the range of apertures that that combination offers:
The exact blur radius varies per Camera/Lens combination, and can be determined with a tool I've made available, as discussed here
The difficulty is that many people also want to control creative sharpening (accentuating or reducing surface structure) with the same sharpening tool, in the same sharpening dialog/session. That's not the optimal route to take. One should preferably use adjustment brushes/layers for additional sharpening/blurring requirements, and controls like Clarity, instead of the Capture sharpening dialog.
The Capture sharpening dialog is for the compensation of Capture process induced blur (=hardware and demosaicing), and the optimal sharpening radius should vary with the aperture that was used. One would expect that the EXIF provides useful info for the Raw converter to present us with better sharpening defaults, but alas ...