Yeah, imagelink is not needed to utilize the smartgun system. Imagelink is basically just a display in your field of vision. You can use it to see AROs, you could even have a little video feed in the upper right corner of your vision with real time images of what your teammate is seeing through their own imaging scope or other cameras they have on them, provided they're transmitting such data to you (or heh, you could just have the big game on or the news or whatever even). But yeah, a gun with no smartgun system will send out data saying how many rounds you have left and what ammo type it is, though without an image link, I don't think you'd be able to see that data in your field of vision.
The smartlink is kinda a specialized version of that- in addition to the ammo left and type, it will display a little crosshairs of where your gun is pointing at, range to target, heat-build up, stress on the parts, etc. Presumably if you use it to fire around corners, it will open up a little video window to show what the camera on the smartgun sees so you can use that to aim and shoot around corners. So if you have a smartlink and no image link, gun data is all you see. If you have an imagelink and a smartgun system on your gun but no smartlink, the gun is sending out that data, but the imagelink doesn't understand what to do with that data without a smartlink.
An image link is definitely useful if you don't have DNI, but it is in no way needed to use a smartgun system. The imaging scope doesn't do jack for a smartgun system unless you have the smartlink modification on it.