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?
This codebase just smells of rot all over.