$authorID = $user->ID;
$authorSlug = $user->user_login;
$authorSlug = mb_strtolower($authorSlug);
'<a href="' . home_url() . '/community/members/' . $authorSlug . '.' . $authorID . '/">' . $authorName . '</a>'
In the legacy code, it is probably best to use XenForo_Link::buildPublicLink with canonical rather than home_url. Also, the user_nicename is what is moved over from XenForo to WordPress. I don't recall why it couldn't be user_login for certain member usernames.
The author_link is used as the hook in XenWord but I'm not sure what it would be for comment author.