TypeScript's type system is unsound in a mathematical sense: https://effectivetypescript.com/2021/05/06/unsoundness/ Though this has little bearing on practicality which is probably why the original comment is at the bottom of the pile.

He's also using a stricter mathematical definition of "type safety" whereas tRPC means it in the colloquial way it's used in the TypeScript ecosystem (that is, a fully typed interface between client and server, ideally that's non-duplicative and inferred directly from your code instead of being manually defined).