without type safety your code is no longer predictable or maintainable

This sounds like someone who's never worked on a large Python project with multiple developers. I've been doing this for almost two decades and we never encounter bugs because of mismatched types.

For reference, the most common bugs we encounter are related to exception handling. Either the code captured the exception and didn't do the right thing (whatever that is) in specific situations or it didn't capture the exception in the right place so it bubbles up waaaaay too high up the chain and we end up with super annoying troubleshooting where it's difficult to reproduce or difficult to track down.

Also, testing is completely orthogonal to types.