The comment has to go somewhere, and IME it makes the common path easy and less common paths not too hard. In particular, to use those functions correctly and understand what they're doing, you often really do just need their name and such an augmented type signature. Having all that information in one place rather than having to extract it out of a docstring (a docstring which might not be shown by default in your editor without additional keystrokes or mouse movements and scrolling), and being able to immediately glance to pieces that don't stick around in short-term memory is nice.

A comment would be fine too, especially if it's right next to the type signature, but to do that you'd need to add extra newlines, and the comment would be in roughly the same spot as the type hint, so I don't know that you gain much. Mypy doesn't really like strings used that way, but mypy isn't a great tool anyway, so c'est la vie?

If somebody just wanted to throw that in a docstring I wouldn't complain though. It's definitely more important that the information exist than that it be in a particular place.