[nullc]

I don't think that (necessarily) says what you think it says.

You can read that as saying that the DL eliminated the background noise rather than saying that the system was conditioned on images of healthy people. From that it may well have been conditioned on just an empty machine or neutral test samples.

If so, there may be a good reason to suspect that it isn't likely to create artifacts that look like or mask anatomical structures.

[cornholio]

You can read it like that, but they surely didn't prove it works like that and the burden of proof is squarely on them.

Realistically, the training set is most likely MRIs of similar tissues and would be naturally biased towards healthy structures. Even the remotest possibility of a hallucination should be addressed and disproved for such an application but they make no mention of it, just "OMG magic ENHANCE button!".