The idea to gather types at runtime is as old as PEP 484. The Dropbox and Facebook teams working on Python type checking know each other. We both worked on our implementations independently since we wanted to first test internally whether the idea holds water. For example, I personally thought it wouldn't be as useful in practice as it turned out to be!

We knew we're going to open-source each others' implementations, esp. that Instagram's is focusing solely on Python 3 which isn't useful for Dropbox at the moment. It just took a while to get through the process of open sourcing what we had (cleaning up the early implementation with limited documentation, decoupling from internal data stores, etc.).

Would it be cheaper if this started out in the open? Probably, but I don't think by quite the margin as you expect.