Which parts of Openapi does it fix?

Looking at the first example - it's far less verbose. Although, seems to be suspiciously minimal, so I can't even tell from a single .tsp route definition what response content type to expect (application/json is the default most likely).

It’s actually human-readable, it has generics, it supports sum and product types in a much more natural way. There’s a lot more, that’s just off the top of my head.

Open api is an exchange format, it quickly become to verbose and repetitive, it is only OK when auto generated and consumed by tooling.