> The speaker basically admitted that static type checking still requires unit tests because statically typed languages have limited granularity in the typing.

Static types only cover one set of bugs, but it is an extremely rewarding set. The payoff of static typing, and what TypeScript focuses on, is early feedback (did I get the name right, did I forget to convert some arguments, what can this value do?). If you focus on the "it eliminates some tests" aspect, you are completely missing the point.

> Personally, when I write a function definition, I always try to visualize the range of possible values that the function will need to handle - So given that I already have that specific range already in my mind as I write the function, it doesn't add any value...

So what the function will handle remain in your head? Or you found a better way to document them than type annotations? Do you write code that anyone else has to read now, or even you have to read later?

> The type definitions are never granular enough to guarantee correctness.

They were never meant for that purpose.