That was very much intentional, I’m actually a bit surprised more people haven’t commented on it.
It shouldn’t really add up or make a lot of sense. The demons have some [i]tricks[/i] up their sleeves (Exador isn’t the only demon to know how to use Abyssal Switch spells) Very little of what you see was built, stuff that’s built tends to be more medieval or older. And as I’ve said some is just crappy facade, some illusion.
The thing to keep in mind is that these higher level demons are OLD, thousands of years old. They make most the vampires in other fiction look like children.
They’ve had a lot of time, a lot of minions and a lot of planes of various tech and magic levels to raid.
Gods don’t usually venture to high tech realms because of the lower mana and too many atheists to bother recruting, but demons that’s another story.
So anyway: lets assume demons are making concrete, they’d make it on some other plane/planet, create prefabricated blocks and zap it in.
Although I really don’t think this happens that much.