My assumption is it's a stealth ban on late night boozing and dancing. In Luxembourg unless you're working or going to hospital etc, the only reason you're out after 11pm is that you're having a good night out as restaurants are shut by 11pm. We also have a rule of 4 in place which limits transmission inside the home.
All that said, given the colder weather and darker mornings I'm not out before 6 and given it's no fun sitting outside bars when it's 4 degrees, I'm not going to bars either. So I can't say I'm even noticing the curfew.