I was a little shocked that they could get it fully working in a week to be honest. My side project is a very similar ambition (https://tsz.dev) but I am in no way claiming success. i keep adding more and more tests to ensure things works. Even after all of TypeScript's own tests pass I am finding bugs which I was totally expecting.
The bar for matching tsc's behavior is really _really_ high. see:
The bar for matching tsc's behavior is really _really_ high. see:
https://github.com/type-challenges/type-challenges
I'm not against using LLMs to write a lot of code. But verification should be 100x more robust now that we can output code at this rate.