The point here is that if you know the number of arguments, calling the function directly is a lot faster than using call_user_func_array(), and this is what call_fuel_func_array() does.
This is still true today, although speed varies between PHP versions, and the method of solving it.
Up to PHP 5.6, it is solved using a check on the number of arguments, and a switch() to call the method directly. From PHP 5.6 onwards, a variadic function call is used (which is faster than using switch).