Yeah, the concept and execution are really impressive but lack of any import is a real blocker I think. Especially in case of spreadsheets - quite often they are not started from scratch, but require previous data or data from other sources etc.
That gap surprised me too. But I couldn't find any way to open a saved document. I think it is just based on the URL.
After exploring it a bit more, I also found that there is no way to load a document that you ask the app to save to your browser IndexDB.