Daniel Oates commented
Evernote does it. Kindle does it. Audible does it. Somehow I think Superbetter could too. I don't use the app as much as I would like because I only connect to the internet when on Wi-Fi. For me this is to help with attention and focus. This app is also a tool that I use for the same purpose. The entire reason I am online now and discovering the forums is because I wanted to make this exact same suggestion. Glad to know I am not alone.
Dominic Aquilina commented
Due to the nature of this system I don't see that as possible. Theoretically they might be able to put a system in place where you sync with an app whenever you connect to the internet, and then any changes would compound, but that would be a development nightmare. I think the system that is currently in place is not suited to offline compatibility.