I did see that tweet. Maybe I was a bit unclear. I mean that the whole editor itself doesn't open when you don't have a connection (at least for me). My reproduction steps are:

1. Open https://stackblitz.com/fork/react

2. Disconnect from internet

3. Refresh the tab

At least for me this gives me a 404. I'm on Chrome. However, when I open the preview tab while offline it doesn't 404 on me.