There's no limit when you upscale with neural nets. Make one good enough and it can fill in the blanks properly.
I would say it can fill in the blanks likely or plausibly. But one can't guarantee that it is the proper or real content, since the neural net is imagining / inventing the upscaled version.
Current neural nets might perhaps not be able to upscale with no limit and still make it look good, but it's not like it can't be done. A good human artist could take a small grainy image and given enough effort draw an upscaled version that would be a very reasonable representation of what a larger image would have looked like, there's no reason to believe that artificial neural networks can't do the same.