Cheap indoor compass
The reason the manufacturer says the compass "doesn't work" indoors is because there can be various magnetic sources indoors.  The compass still works fine telling you which direction the magnetic field north lines go, but that direction may have little to do with the earth's magnetic field.

Since these magnetic source are quite localized, the apparent direction can change significantly as the car drives around on the floor of a single room.  This makes such a compass useless for navigation.

However, turning in place might still work.  The car wouldn't be changing location, so whatever magnetic field is around it should be fairly consistent as the car turns in place.  You might get away with stopping the car, looking at the magnetic field direction, turning in place some fixed amount relative to the magnetic field, then moving again after the turn.  Doing a broad turn while also moving is less likely to work as expected.