This seems to be using a hack I also figured out back when I was in high school, working on a project. I also wrote about it years later on my blog[0]. If you look at the source it creates an event with an sms remainder but the time of the event and the scheduled time of the remainder are created in such way that the remainder is sent immediately. There's also a low limit on the number of characters you can use (~60).
[0]: http://www.randomshouting.com/2011/02/06/Free-SMS-notificati...