All the redirecting is performed by the zero duplicate
. The code doing this is in index.php.
The zero dupe only redirects example.com/phpBB/ (and example.com/phpBB/index.php) to example.com/phpBB/forum.html if you are using forum.html as you forum index url, but you could also use "nothing" and see example.com/phpBB/index.php and example.com/phpBB/forum.html redirected to example.com/phpBB/.
So, if you see example.com/phpBB/ being redirected to example.com/phpBB/forum.html, it may be because it is required for some option (such as virtual root) or because you have chosen it. In such case, the .htaccess generator should add the appropriate rewriterule to handle it. But it's not the rewriterules that will do the actual redirect.