It doesn’t denote quality but quality is the connotation.

In my interpretation, it indicates that the picture contains at least 4000 pixels belonging to the moon.

Any reasonable reader will assume that the Moon has to fill a significant portion of the frame and be well exposed as well as sharp for this headline to be meaningful.

I agree it’s not precise and I also agree it’s not a generally very useful way to talk about resolution, but I think the authors were looking for a way to convey high resolution and quality while using terms anyone might have heard and have a rough understanding of.