It was an exaggeration to say he "found" Riverene, but he confirmed that it should be where he thought it was.
There were maps at the end of the first book Gio translated that showed the entire landscape of Azaereal that Leonidas' group had explored.
The mountain they were currently on was one of the only mountains in the entire region. By checking the surrounding area, Gio confirmed where it was on the map and sectioned off their location.
Essentially, their current position in the monster world could be transposed directly onto New Renis if the map of Azaereal was overlaid with the map of the old Earthen country.
The two areas were placed next to each other for a reason. This region was moved north simply because the two sceneries couldn't exist together.
