But he did get a runtime failure. Imagine this wasn't about adding a function, but changing an existing function that is being called from 23 different location

But he did get a runtime failure. Imagine this wasn't about adding a function, but changing an existing function that is being called from 23 different locations in a 100kloc codebase, and that it contains some tricky condition logic so that a bug might only be triggered in 5% of the calls. Without exhaustive unit tests, a type error potentially would not show up until all those call sites had been executed with the specific arguments required to trigger the bug. His point was that in a statically typed language, this would all be detected at compile time.