Considering that most of their software has been developed for decades and AI assistants have only started becoming useful in the last ~4 years it would be very surprising if 30% of their code is AI written. I doubt they even touched 30% of their code in the last 4 years. But what is perfectly plausible is that 30% of their code is written by code generators. Microsoft has a lot of interface code. All the windows DLLs that are just thin syscall interfaces, the COM and OLE interfaces in their office suite and everywhere else, whatever Office uses nowadays for interoperability to allow you to embed content of one product in another, whatever APIs their online products use, etc. In the leaked Windows XP source code it can be difficult to find the actual source code in between the boilerplate files containing repeated definitions, and in the decades since then the world has only leaned more into code generation.
"I’d say maybe 20%, 30% of the code that is inside of our repos today and some of our projects are probably all written by software".
> He knows exactly what he is saying
Very much so. He's plainly pulled those figures out of his arse, keeping it vague so he can't be blamed later for lying, for nothing more than "look how much we use AI" marketing purposes.