For bots or users who don’t have permission to view profiles, it doesn’t make much sense to allow the links to exist to go to the viewprofile pages.
But also, it is possible to give Guests permission to view profiles, but in reality that’s impossible since it’s restricted from Guests hardcoded into the function.
This would be my suggestion: (in get_username_string() function)

if ($user_id && $auth->acl_get('u_viewprofile')) // globalize $auth    {        …    }    else

Or at least...

if ($user_id && $user_id != ANONYMOUS && !$user->data['is_bot'])    {        …    }    else

So the links won’t show unnecessarily for spiders and bots.