There's a very strong likelihood that site is a scam site Flash. They're borrowing your browser to help them find the key. If you want to know more about the 1000btc puzzle which was originally set up in 2015, either go to the original post on bitcointalk forum here:

https://bitcointalk.org/index.php?topic=5218972.0

or check out this medium article on it for a summary.

https://medium.com/@deepml1818/the-1000-btc-puzzle-a-deep-dive-for-newcomers-and-crypto-veterans-c41b89688ce8