If you use Swift and consume the library in an Objective-C project (which is perfectly possible) you will add the Swift ABI to your project automatically which

If you use Swift and consume the library in an Objective-C project (which is perfectly possible) you will add the Swift ABI to your project automatically which adds about 10MB to your project. The ABI needs to be added because the ABI isn't stable yet. For mobile apps that's still a big consideration. The other way around is much easier, that's why he proposes Swift wrappers around Objective-C code that allow a Swifty interface with your libraries. But often using generics and nullability will already help a lot.