But one of its problems surely is "horrible error messages", and this project addresses it, too.
However, it doesn’t address the atrocious compile times, excessively verbose syntax and fragile macro system.
The thing is, my goal was to enhance the text editor and improve the error messages, but not to abandon TeX. That's the price I have to pay.
Types are just the mechanism. The real problem is the one fph pointed out: LaTeX reports errors in its own terms instead of the author’s, and review comments live somewhere else. Gradual typing is simply how the compiler can understand the names you introduce without pretending that all of LaTeX can be typed, because it can't.