Wow great discovery there ! I always thought if the site owners used its visitors as a way to search for bitcoin addresses with some balance for THEM and it turned out to be true .
BTW, the site : btckey.space is run by Jude Austin(https://bitcointalk.org/index.php?action=profile;u=105076) , its official thread is : https://bitcointalk.org/index.php?topic=1005950.100. If your findings are correct, then post on its thread and lets wait for the owners reply.