Specifically- it appears to be mostly C99, lots of macros, lots of ugly buffer manipulation. But it's tooled for PREFast so you know it's secure! (https://github.com/microsoft/omi/blob/4ce2cf1cb0aa656b8eb934...) (j/k... I haven't seen that in ages). Guessing a port of MS's internal WMI codebase?
That is why there is "Modern C", "Modern C++" as talked at conferences, blog posts and endless over here, and then real life code that we actually find out in production.
Ironically, most modern C++ code I wrote was not for Microsoft, but a much smaller company that is not in any way involved in ISO C++ standardization process. It was due to that company culture - senior engineering leadership (which were also the founders) were all big standard C++ fans, and they wrote much of the original codebase.
Large companies like MS often have codebases that are so ancient (e.g. HRESULTs and IF_ERR_GOTO type macros everywhere), they only get modernized very gradually if at all.
I only use C++ for libraries that are then plugged into managed runtimes.
Most of the stuff I see in the wild could be considered what I call C+, basically MFC style with plenty of C love.
While Microsoft has the excuse for older code bases, stuff like WRL and C++/WinRT also has its share of not only following what is preached regarding C++ Core Guidelines love.
This codebase just smells of rot all over.