It is not "modern" C++.
It uses GENie, a fork of premake.
https://github.com/bkaradzic/GENie
Appreciating that CMake has its warts, it also has a ton of mindshare and has lots of convenient modules for handling common dependencies. What are the motivations to use a Lua-based scheme instead?
CMake is "too complicated" and "you need to be an expert". Understandable, I suppose. There are certainly specific things in CMake which are pretty terrible, like the add_custom_command/add_custom_target dance, but from the perspective of someone who has had to become an expert in it (via ROS/catkin), I would be unlikely to give it up. There's just way too much stuff it gives you for free, especially when it comes to things like packaging, testing, etc.
GENie seems to be focused first and foremost on a project/solution-oriented IDE workflow, with the Makefile generator as the one that's tacked on. So I can definitely appreciate that if you're working on a project where everyone's in an IDE anyway, it would make sense to use a generator that has the IDE's concepts as a first class citizen.