Nowadays Typescript becomes a defacto language for SPA frameworks and the React authors also recommend using it for better code quality and extendibility.
You can have a look at this post to quickly get used to Typescript. Please give a try, Typescript is awesome .
You should mention it clearly. I also purchased it for a new project and now I am seeing that it is in typescript. Please let me know where in the documentation you have mentioned that the metronic 8 comes with typescript for react.