I think part of the problem is "developer productivity" is a poorly-defined term that means different things to different people.
To some, it means getting something minimal working and running as quickly as possible, accepting that there will be bugs, and that a comprehensive test suite will have to be written later to suss them all out.
To others (myself included), it means I don't mind so much if the first running version takes a bit longer, if that means the code is a bit more solid and probably has fewer bugs. And on top of that, I won't have to write anywhere near as many tests, because the type system and compiler will ensure that some kinds of bugs just can't happen (not all, but some!).
And I'm sure it means yet other things to other people!
To some, it means getting something minimal working and running as quickly as possible, accepting that there will be bugs, and that a comprehensive test suite will have to be written later to suss them all out.
To others (myself included), it means I don't mind so much if the first running version takes a bit longer, if that means the code is a bit more solid and probably has fewer bugs. And on top of that, I won't have to write anywhere near as many tests, because the type system and compiler will ensure that some kinds of bugs just can't happen (not all, but some!).
And I'm sure it means yet other things to other people!