A lot of the guts of it are still the same ones that have been in C/C++ from the "dawn of time" of calc.exe. One thing to notice, for instance is that the "RatPack" infinite-precision rational library in the middle of the repository has a Copyright date specifically in it of 1995 and that seems credible.

(Also, it's C++/CX which is similar in syntax but different in runtime to Managed C++/CLI. C++/CX produces WinRT-flavor COM bindings, not CLR-intended IL code. It sounds like some folks are hoping to migrate it, now that it is open source and could be done as a community effort, to even more modern C++/WinRT which uses modern C++ features and does away with the need of special syntax.)

Oh you're right it is c++/cx. I saw that carrot pointer and jumped to conclusions.