This was quick enough to add to the core libraries:
I have to say that I’m also surprised by how different the floor area is when you account for the wall thickness. So I can see your point, @MaxMarschall .
I will add a component to the LBT Grasshopper plugin that exposes this method soon. In the meantime, you can access it using this method in the honeybee-energy library:
room.properties.energy.floor_area_with_constructions('Meters')
You might just have to replace 'Meters' with the units system of your Rhino model.