In the 2nd paragraph, the tutorial explains that the positive leg of an LED is called the anode, and that the negative leg of an LED is called the anode.
definitely confusing 
To build the circuit, grab an LED and attach its long, positive leg (called the anode) to pin 13. Attach the short, negative leg (called the anode) to ground.
Hey, meet my brother Larry, and my other brother named Larry.
Lefty
Thought your brothers were called Lefty and Lefty
If you cut the long lead shorter would both leads become cathodes?
Don
More serious in the code example is not using unsigned long for the timer variable. The interval variable could be const as it does not change.
Also, between the last two closing braces in the example, I propose (due to discussion held elsewhere on TWO non-delayed LEDs blinking) thet a comment is added.
}
/* Other code that needs to execute goes here.
It will be called many thousand times per second because the above code
does not wait for the LED blink to finish. */
}