Yes it says they are "rewriting the core in TypeScript", but that does not necessarily mean changing the code in any meaningful way. It means adding static types to the core itself, not only the interface (which DefinitivelyTyped is for).
Re:Next.js 8 released
by timneutkens on Monday February 11, 2019 @01:12PM
(#19136652)
Max is definitely right here. We're not planning to deprecate or "completely rewrite" anything.
We're not even "rewriting everything in Typescript at once". We're slowly converting files over one by one. No public APIs are changed. Furthermore, we have an extensive test suite of end-to-end tests for both development and production behavior.
The parent says something different and if it were just about adding types then you need just to add a file to DefinitelyTyped which I guess happened already.