Let's think about what happens on a team: If a single person goes and reads the source code to learn how the public API works instead of simply using the exported types, that's a wasted thirty minutes instead of two - okay, fine, whatever. If all of my engineers have to do this, suddenly each of them is spending that amount individually; and just like that, we've wasted an entire man-day, for nothing.

As you declared in your own comment, TypeScript is great for reading, hard for writing. I don't know about you, but my code is read several times more often, and by different people, than written. TypeScript is a no-brainer, time-saving wise.