The webpage contain the following:

Treasure of the Pirates!
It should in some of this (or hidden) island!

Hint: you need a map of this place to find it!

Island #0
Island #1
Island #9998
Island #9999

We are looking for the map. We check robots.txt: it contains: Sitemap: /treasuremap.xml

The file /treasuremap.xml contain the location of the "treasure":


And the "treasure" contain the flag: flag{tr3asur3_hunt1ng_fUn}

Original writeup (https://maggick.fr/2020/04/hackpack-ctf-2020.html).