They could have just used flash
The language is very familiar for AS3 devs and it transpiles to performant C++.
Many games such as Papers Please have been made with Haxe.
There are numerous engines/libs for game devs:
The quality is so very high, much much higher than a typical open source project, because it's not just a bit of code, but an entire toolchain and workflow. So much so that an entire engine is built off this framework! https://armory3d.org/
I also tried Heaps which on paper looks amazing but could not proceed due to lack of documentation.