BigGreenGilbert; He didn't "rethread" it, he cut new thread.
I said re-thread because cutting a new thread can alter the dimension of the original bolt thread which you wouldn't want as it can effect the nuts ability to lock and weakens the bolt, as indeed lengthening the thread does, but hey ho, you can get none split dies that are designed to clean up damaged or bruised threads as in re-threading while not effecting the dimensions and can also be used to cut new thread..
and example of
https://www.grainger.com/product/CLE...xed-Die-20X681
..