There are many different ways to use the Aeropress. I'd assume that the recipe the author is following simply asks for a 30 second steep time.

I personally found that the time actually doesn't matter that much, you control extraction by grind size, water temperature and agitation. It might be that if you grind too fine you can still reduce extraction by cutting the time short, but that seems rather inconvenient for this method.

I usually let it steep for 5 minutes, but the exact time doesn't change much. Shorter times aren't that desirable for me anyway as the coffee is still too hot then as I start with boiling water.