First, hiatal hernias are a normal aberration in mature patients. There is no definitive connection between mild to moderate hernias and reflux.
Also, it's common for hernias to be noted during an EGD because the stomach is inflated during the procedure, and if the surgeon adds too much air...voila!...instant hernia. This is true in my case, and I have what they call a "sliding hernia" because it can move beyond the diaphragm, then back.
There may be no apparent hernia present during a Nissen, but as a matter of course, surgeons add a few stitches to tighten the aperture through the diaphragm to secure the surgical site from migrating above that zone into the chest cavity.