This is not the whole story.
Long ago, there was a major foul-up in the design of the Arduino NG, the predecessor to the Diecimila which preceded the Duemilanove from which the UNO was derived!
As you will note, pin 13 was connected to a resistor and LED, but instead of also connecting the pin to the header, the header instead connected to the LED and that via a second 470 Ohm resistor, not only rendering pin 13 essentially functionally inaccessible to the outside world but even then, while this meant that an external LED could safely be connected without a resistor to this header pin, its brightness was minimised
Quite a peculiar blunder - I do wonder how it occurred (in two successive issues!), back in the mists of Arduino time.