I think his point is more that someone who's been exposed to a language for years but isn't interested is going to pay off much less in the long run than someon

I think his point is more that someone who's been exposed to a language for years but isn't interested is going to pay off much less in the long run than someone who's using it for the first time but enjoys learning. The reason he mentioned a few weeks of training for the language meant that they would be able to start making changes within a few weeks and be making major contributions by a few months. The fact that their first month or so may be relatively unproductive is less of a concern if they're constantly getting better and better. That said, a few weeks of training plus exposure to a large codebase in a language should be enough for even the short term unless they're trying to learn a whole new paradigm or some huge frameworks/libraries.