I can guarantee you there is more publicly accessible javascript in the world than C++.
Copilot will autocomplete entire functions as well, sometimes without comments or even after just typing "f". It uses your previous edits as context and can assume what you're implementing pretty well.
I can guarantee you that the author was referencing code within Google. That is, their tooling is trained off internal code bases. I am imagining c++ dwarfs javascript.
They're probably talking about Closure Compiler type annotations [0], which never really took off outside Google, but (imo) were pretty great in the days before TypeScript. (Disclosure: Googler)
Copilot will autocomplete entire functions as well, sometimes without comments or even after just typing "f". It uses your previous edits as context and can assume what you're implementing pretty well.