The Paris scene is not 'geolocated' and thus has no Scene Coordinate System set. This is usually done when importing something geolocated.
The default case in with the KML exporter is that there's a SCS set, thus in your case it asks specifically.
The terrain exporter does not ask about a SCS, but if it's set it will use it. [We may adapt this behavior in future.]
What I'd recommend is that you set the SCS in the scene, then test the exporter again, but note that the position is not set correctly, thus the models will not show up in 'actual France'. :)
Does that make sense ?