alt Hacker NewsABI is the language used to write the OS, thus OP is kind of right.
While Windows has moved away from pure C, and nowadays has ABIs across C, C++, .NET, COM, WinRT interfaces, you can still program Windows applications in straight C.
The caveat is to only use APIs up to Windows XP, and Petzold's book to follow along.
They’re describing higher level API that may have a separate ABI than the lower level system.
But like I said, they’re conflating the lower level ABI with the higher level API/ABI.
All the systems they mentioned have an equal C ABI available for talking to the core system.