Discalimer: my electronics knowledge is 40 years old and I'm slowly getting some of it back.
The LEDs are just normal LEDs. My recommendation would be to follow the directions and use a resistor in series.
If you want in-depth information, google 'raspberry pi led internal resistors and gpio' you will find some technical discussions, examples both ways, but it won't hurt to add the external resistor (even to the Piper circuits) and it just might prevent you from frying the Pi - which will make you sad, at least it made me sad when I fried one of mine. (I fried a Pi Hat circuit board too ) but now I have one i can let people touch and see what it looks like and I don't care if you dorp it or spill something on it etc)