They go on to say you “can do all your writing anytime, anywhere on this website, as long as you have a computer and access to internet”, that their metric is 750 words instead of 3 pages, and they require an account to use the service. I don’t see why you wouldn’t just do it in any semi-competent text editor, without needing an internet connection or another online account.
Finally, they end by suggesting the “don’t break the chain” productivity method[2], saying “it is inspired by a productivity trick invented by Jerry Seinfeld. You can google it for more detail”. If you google it, you might be quick to find out from the horse’s mouth that’s a common misconception, as Seinfeld did not create the system[3].
[1]: https://www.theguardian.com/lifeandstyle/2014/oct/03/morning...
[2]: https://lifehacker.com/jerry-seinfelds-productivity-secret-2...
[3]: https://www.reddit.com/r/IAmA/comments/1ujvrg/jerry_seinfeld...