It's not the website that has been copied. They are simply powered by the same platform. At the bottom of the page of this forum you will see
Powered by phpBB 2.0.1 © 2001 phpBB Group
At the bottom of the page of that other one you will find
Powered by phpBB 2.0.2 © 2001, 2002 phpBB Group
Looks like a slightly different version, but basically the same.