I'm deeply skeptical of the "X reimplemented and it was super easy" thing.

The devil is in the detail.

So many edge cases unlikely to be there.

So many details or fine details unlikely to be there.

Years of bug fixes.

If it is literally a drop in replacement and it passes all the tests, and you're replicating something with and extremely thorough test suite, then sure I'll give you the benefit of the doubt.

Otherwise, I don't believe people "rebuilt X product in a week".

I don't necessarily buy it either, but TFA talks about the test suite. They basically pulled 2k unit tests and 400 E2E tests from Next and made sure they all passed.