A big part of TS is the ease of transition from JS where you immediately get benefits through gradual typing and return type inference is probably a big part of that.
The github comment says: " (I might suggest the underlying problem in this code is relying on inference too much, but the threshold for "too much" is difficult to communicate to users.)"
The first part is exactly right; the second part is well established in the Rust language spec and Haskell style guides like http://www.cis.upenn.edu/~cis552/current/styleguide.html and https://kowainik.github.io/posts/2019-02-06-style-guide#func...