CoreVideo and CoreAudio are both implemented in C on Apple systems. There are higher level APIs like AVFoundation implemented in Obj-C/Swift, but the codecs themselves are written in C. Even the mid-level frameworks like AudioToolbox and VideoToolbox are written in C. I’m not as familiar with Microsoft but imagine it’s similar.
Also the article doesn’t actually mention OOP. You can use polymorphism without fully buying into OOP (like Go does).
The great thing about C is its interoperability, which is why it’s the go to language for things like codecs, device drivers, kernel modules, etc.
Secondly, Apple and Microsoft, do just fine with Objective-C and C++ for their video codecs, without having to manually implement OOP in C.