The small thread should also not go through the hole :)
Place the flat side of the thread on the shelf above the hole and screw in the bulb holder from below.
Then you can screw in the bulb...
I may present the solution myself: It is because Galaxus initially recommended the 'Eglo Vintage Amber (E27, 330lm, 4W)' for this luminaire, but it is not dimmable. It works with a dimmable LED bulb.