The idea here is that unused land in a city should be highly taxed, but unused land in rural areas should not.

The point of Georgism is to get rid of the 'unused' part. You're just taxed on land, full stop, based on the most valuable things that could probably be done with it right now.

Land in a city center? Hefty taxes, identical for a parking lot and a skyscraper, because the parking lot could be a skyscraper instead.

Land in a low population rural area? Low taxes, because the likely uses are homes or businesses that could be built in any other rural area.