That's a cool experiment, but from a quick read they are creating a compiler which is a bit different than web development. Looking at the related work all seem to contradict this finding and suggest preference to statically typed languages, such as one that looked at API development.

From my own experience with dynamic types it's not the dev time that increases but maintenance time. When you have a team working on an evolving codebase with data types distributed in caches, databases, and code it seems inevitable that you will have an error if you don't do any type checks. Even good linters can miss something that would have been easy to catch with static type checks.