Both you and Smasher have good thoughts.
Calling it "her" does not personify the "earth" into meaning "planet earth". It's a very modern interpretation to think of Earth as one of many planets not in the center the universe, or even as something not flat.
Though many things are referred to as her, whether it be a gun or a ship, those are still specific things. The earth is a very general and unidentified plane of existence.
The earth goddess is in every ancient mythology. This is before earth was anything more than an infinite plane which had water and life on it. (It certainly was not thought of in the abstract concepts of planets and being mostly water covered, etcetera.) Therefore, the goddess of the ground is the same as the goddess of this planet, for the two are entirely synonymous.
That's why this Heron Guard refers to the earth as "her." There isn't any differentiation in ancient or primitive conceptions of the world between "earth" and "Earth." If the flavor said "Earth," it would indicate that he'd be talking about the metaphorical representation of the life-giving soil.
