Personally, I have no idea why libgpsbt ("bt" standing for "Bluetooth") works with the internal GPS receiver. I think it might be a throwback from libgpsmgr, which worked on both serial port devices (which the internal GPS receiver acts like?) and bluetooth port devices (i.e. RFCOMM). Perhaps libgpsbt, which uses libgpsmgr, includes serial port support for that reason.

The default setting in Maemo Mapper is "Bluetooth" with an empty MAC address, though, so if you don't modify the settings, Maemo Mapper should "just work" on the N810. The documentation also recommends that configuration for the N810. I agree that the label is confusing, though.