You misunderstood. Game developers should use a _good_ third–party library, not a _bad_ one. At a minimum they should be able to read the source code so that they know it is good. Thus open source libraries should be at the top of the list.

If you don't know what “good” looks like, take a look at [Serde](https://serde.rs/). It’s for Rust, but its features and overall design are something you should attempt to approach no matter what language you’re writing in.